diff --git a/app/client/components/CellSelector.ts b/app/client/components/CellSelector.ts
new file mode 100644
index 00000000..2d4d38d2
--- /dev/null
+++ b/app/client/components/CellSelector.ts
@@ -0,0 +1,157 @@
+import ko from 'knockout';
+import type BaseView from 'app/client/components/BaseView';
+import type {DataRowModel} from 'app/client/models/DataRowModel';
+import {between} from 'app/common/gutil';
+import {Disposable} from 'grainjs';
+
+export const ROW = 'row';
+export const COL = 'col';
+export const CELL = 'cell';
+export const NONE = '';
+
+export type ElemType = 'row' | 'col' | 'cell' | '';
+
+interface GridView extends BaseView {
+  domToRowModel(elem: Element, elemType: ElemType): DataRowModel;
+  domToColModel(elem: Element, elemType: ElemType): DataRowModel;
+}
+
+export class CellSelector extends Disposable {
+  // row or col.start denotes the anchor/initial index of the select range.
+  // start is not necessarily smaller than end.
+  // IE: clicking on col 10 and dragging until the mouse is on col 5 will yield: start = 10, end = 5
+  public row = {
+    start: ko.observable(0),
+    end: ko.observable(0),
+    linePos: ko.observable('0px'),    // Used by GridView for dragging rows
+    dropIndex: ko.observable(-1),     // Used by GridView for dragging rows
+  };
+  public col =  {
+    start: ko.observable(0),
+    end: ko.observable(0),
+    linePos: ko.observable('0px'),    // Used by GridView for dragging columns
+    dropIndex: ko.observable(-1),     // Used by GridView for dragging columns
+  };
+  public currentSelectType = ko.observable<ElemType>(NONE);
+  public currentDragType = ko.observable<ElemType>(NONE);
+
+  constructor(public readonly view: GridView) {
+    super();
+    this.autoDispose(this.view.cursor.rowIndex.subscribe(() => this.setToCursor()));
+    this.autoDispose(this.view.cursor.fieldIndex.subscribe(() => this.setToCursor()));
+    this.setToCursor();
+  }
+
+  public setToCursor(elemType: ElemType = NONE) {
+    // Must check that the view contains cursor.rowIndex/cursor.fieldIndex
+    // in case it has changed.
+    if (this.view.cursor.rowIndex) {
+      this.row.start(this.view.cursor.rowIndex()!);
+      this.row.end(this.view.cursor.rowIndex()!);
+    }
+    if (this.view.cursor.fieldIndex) {
+      this.col.start(this.view.cursor.fieldIndex());
+      this.col.end(this.view.cursor.fieldIndex());
+    }
+    this.currentSelectType(elemType);
+  }
+
+  public containsCell(rowIndex: number, colIndex: number): boolean {
+    return this.containsCol(colIndex) && this.containsRow(rowIndex);
+  }
+
+  public containsRow(rowIndex: number): boolean {
+    return between(rowIndex, this.row.start(), this.row.end());
+  }
+
+  public containsCol(colIndex: number): boolean {
+    return between(colIndex, this.col.start(), this.col.end());
+  }
+
+  public isSelected(elem: Element, handlerName: ElemType) {
+    if (handlerName !== this.currentSelectType()) {
+      return false;
+    }
+
+    // TODO: this only works with view: GridView.
+    // But it seems like we only ever use selectors with gridview anyway
+    const row = this.view.domToRowModel(elem, handlerName);
+    const col = this.view.domToColModel(elem, handlerName);
+    switch (handlerName) {
+      case ROW:
+        return this.containsRow(row._index()!);
+      case COL:
+        return this.containsCol(col._index()!);
+      case CELL:
+        return this.containsCell(row._index()!, col._index()!);
+      default:
+        console.error('Given element is not a row, cell or column');
+        return false;
+    }
+  }
+
+  public isRowSelected(rowIndex: number): boolean {
+    return this.isCurrentSelectType(COL) || this.containsRow(rowIndex);
+  }
+
+  public isColSelected(colIndex: number): boolean {
+    return this.isCurrentSelectType(ROW) || this.containsCol(colIndex);
+  }
+
+  public isCellSelected(rowIndex: number, colIndex: number): boolean {
+    return this.isColSelected(colIndex) && this.isRowSelected(rowIndex);
+  }
+
+  public onlyCellSelected(rowIndex: number, colIndex: number): boolean {
+    return (this.row.start() === rowIndex && this.row.end() === rowIndex) &&
+      (this.col.start() === colIndex && this.col.end() === colIndex);
+  }
+
+  public isCurrentSelectType(elemType: ElemType): boolean {
+    return this._isCurrentType(this.currentSelectType(), elemType);
+  }
+
+  public isCurrentDragType(elemType: ElemType): boolean {
+    return this._isCurrentType(this.currentDragType(), elemType);
+  }
+
+  public colLower(): number {
+    return Math.min(this.col.start(), this.col.end());
+  }
+
+  public colUpper(): number {
+    return Math.max(this.col.start(), this.col.end());
+  }
+
+  public rowLower(): number {
+    return Math.min(this.row.start(), this.row.end());
+  }
+
+  public rowUpper(): number {
+    return Math.max(this.row.start(), this.row.end());
+  }
+
+  public colCount(): number {
+    return this.colUpper() - this.colLower() + 1;
+  }
+
+  public rowCount(): number {
+    return this.rowUpper() - this.rowLower() + 1;
+  }
+
+  public selectArea(rowStartIdx: number, colStartIdx: number, rowEndIdx: number, colEndIdx: number): void {
+    this.row.start(rowStartIdx);
+    this.col.start(colStartIdx);
+    this.row.end(rowEndIdx);
+    this.col.end(colEndIdx);
+    // Only select the area if it's not a single cell
+    if (this.colCount() > 1 || this.rowCount() > 1) {
+      this.currentSelectType(CELL);
+    }
+  }
+
+  private _isCurrentType(currentType: ElemType, elemType: ElemType): boolean {
+    console.assert([ROW, COL, CELL, NONE].indexOf(elemType) !== -1);
+    return currentType === elemType;
+  }
+}
diff --git a/app/client/components/GridView.js b/app/client/components/GridView.js
index 39a86760..f16e81d0 100644
--- a/app/client/components/GridView.js
+++ b/app/client/components/GridView.js
@@ -1,4 +1,4 @@
-/* globals alert, document, $ */
+/* globals alert, $ */
 
 const _         = require('underscore');
 const ko        = require('knockout');
@@ -19,8 +19,9 @@ const commands      = require('./commands');
 const viewCommon    = require('./viewCommon');
 const Base          = require('./Base');
 const BaseView      = require('./BaseView');
-const selector      = require('./Selector');
+const selector      = require('./CellSelector');
 const {CopySelection} = require('./CopySelection');
+const {SelectionSummary} = require('./SelectionSummary');
 const koUtil      = require('app/client/lib/koUtil');
 const convert       = require('color-convert');
 
@@ -39,6 +40,7 @@ const {setPopupToCreateDom} = require('popweasel');
 const {CellContextMenu} = require('app/client/ui/CellContextMenu');
 const {testId} = require('app/client/ui2018/cssVars');
 const {contextMenu} = require('app/client/ui/contextMenu');
+const {mouseDragMatchElem} = require('app/client/ui/mouseDrag');
 const {menuToggle} = require('app/client/ui/MenuToggle');
 const {showTooltip} = require('app/client/ui/tooltips');
 const {parsePasteForView} = require("./BaseView2");
@@ -79,11 +81,9 @@ function GridView(gristDoc, viewSectionModel, isPreview = false) {
   this.scrollTop = ko.observable(0);
   this.isScrolledTop = this.autoDispose(ko.computed(() => this.scrollTop() > 0));
 
-  this.cellSelector = this.autoDispose(selector.CellSelector.create(this, {
-    // This is a bit of a hack to prevent dragging when there's an open column menu
-    // TODO: disable dragging when there is an open cell context menu as well
-    isDisabled: () => Boolean(!this.ctxMenuHolder.isEmpty())
-  }));
+  this.cellSelector = selector.CellSelector.create(this, this);
+  this.selectionSummary = SelectionSummary.create(this,
+    this.cellSelector, this.tableModel.tableData, this.sortedRows, this.viewSection.viewFields);
   this.colMenuTargets = {}; // Reference from column ref to its menu target dom
 
   // Cache of column right offsets, used to determine the col select range
@@ -856,7 +856,6 @@ GridView.prototype._getColStyle = function(colIndex) {
   return { 'width' : this.viewSection.viewFields().at(colIndex).widthPx() };
 };
 
-
 // TODO: for now lets just assume you are clicking on a .field, .row, or .column
 GridView.prototype.domToRowModel = function(elem, elemType) {
   switch (elemType) {
@@ -1359,19 +1358,19 @@ GridView.prototype.rowMouseDown = function(elem, event) {
   }
 };
 
-GridView.prototype.rowMouseMove = function(elem, event) {
+GridView.prototype.rowMouseMove = function(event) {
   this.cellSelector.row.end(this.currentMouseRow(event.pageY));
 };
 
-GridView.prototype.colMouseMove = function(elem, event) {
+GridView.prototype.colMouseMove = function(event) {
   var currentCol = Math.min(this.getMousePosCol(event.pageX),
                             this.viewSection.viewFields().peekLength - 1);
   this.cellSelector.col.end(currentCol);
 };
 
-GridView.prototype.cellMouseMove = function(elem, event, extra) {
-  this.colMouseMove(elem, event);
-  this.rowMouseMove(elem, event);
+GridView.prototype.cellMouseMove = function(event) {
+  this.colMouseMove(event);
+  this.rowMouseMove(event);
   // Maintain single cells cannot be selected invariant
   if (this.cellSelector.onlyCellSelected(this.cursor.rowIndex(), this.cursor.fieldIndex())) {
     this.cellSelector.currentSelectType(selector.NONE);
@@ -1388,58 +1387,70 @@ GridView.prototype.createSelector = function() {
 // but we can't attach any of the mouse handlers in the Selector class until the
 // dom elements exist so we attach the selector handlers separately from instantiation
 GridView.prototype.attachSelectorHandlers = function () {
-  // We attach mousemove and mouseup to document so that selecting and drag/dropping
-  // work even if the mouse leaves the view pane: http://news.qooxdoo.org/mouse-capturing
-  // Mousemove/up events fire to document even if the mouse leaves the browser window.
-  var rowCallbacks =  {
-    'disableDrag': this.viewSection.disableDragRows,
-    'mousedown': { 'select': this.rowMouseDown,
-                   'drag': this.styleRowDragElements,
-                   'elemName': '.gridview_data_row_num',
-                   'source': this.viewPane,
-    },
-    'mousemove':  { 'select': this.rowMouseMove,
-                    'drag': this.dragRows,
-                    'source': document,
-    },
-    'mouseup':   { 'select': this.rowMouseUp,
-                   'drag': this.dropRows,
-                   'source': document,
-    }
-  };
-  var colCallbacks =  {
-    'mousedown': { 'select': this.colMouseDown,
-                   'drag': this.styleColDragElements,
-                   // Trigger on column headings but not on the add column button
-                   'elemName': '.column_name.field:not(.mod-add-column)',
-                   'source': this.viewPane,
-    },
-    'mousemove':  { 'select': this.colMouseMove,
-                    'drag': this.dragCols,
-                    'source': document,
-    },
-    'mouseup':   { 'drag': this.dropCols,
-                   'source': document,
+  const ignoreEvent = (event, elem) => (
+    event.button !== 0 ||
+    event.target.classList.contains('ui-resizable-handle') ||
+    // This is a bit of a hack to prevent dragging when there's an open column menu
+    // TODO: disable dragging when there is an open cell context menu as well
+    !this.ctxMenuHolder.isEmpty()
+  );
+
+  this.autoDispose(mouseDragMatchElem(this.viewPane, '.gridview_data_row_num', (event, elem) => {
+    if (!ignoreEvent(event, elem)) {
+      if (!this.cellSelector.isSelected(elem, selector.ROW)) {
+        this.rowMouseDown(elem, event);
+        return {
+          onMove: (ev) => this.rowMouseMove(ev),
+          onStop: (ev) => {},
+        };
+      } else if (!this.viewSection.disableDragRows()) {
+        this.styleRowDragElements(elem, event);
+        return {
+          onMove: (ev) => this.dragRows(ev),
+          onStop: (ev) => this.dropRows(),
+        };
+      }
     }
-  };
-  var cellCallbacks =  {
-    'mousedown': { 'select': this.cellMouseDown,
-                   'drag' : function(elem) { this.scheduleAssignCursor(elem, selector.NONE); },
-                   'elemName': '.field:not(.column_name)',
-                   'source': this.scrollPane
-    },
-    'mousemove':  { 'select': this.cellMouseMove,
-                    'source': document,
-    },
-    'mouseup':   { 'select': this.cellMouseUp,
-                   'source': document,
+  }));
+
+  // Trigger on column headings but not on the add column button
+  this.autoDispose(mouseDragMatchElem(this.viewPane, '.column_name.field:not(.mod-add-column)', (event, elem) => {
+    if (!ignoreEvent(event, elem)) {
+      if (!this.cellSelector.isSelected(elem, selector.COL)) {
+        this.colMouseDown(elem, event);
+        return {
+          onMove: (ev) => this.colMouseMove(ev),
+          onStop: (ev) => {},
+        };
+      } else {
+        this.styleColDragElements(elem, event);
+        return {
+          onMove: (ev) => this.dragCols(ev),
+          onStop: (ev) => this.dropCols(),
+        };
+      }
     }
-  };
+  }));
 
-  this.cellSelector.registerMouseHandlers(rowCallbacks, selector.ROW);
-  this.cellSelector.registerMouseHandlers(colCallbacks, selector.COL);
-  this.cellSelector.registerMouseHandlers(cellCallbacks, selector.CELL);
-};
+  this.autoDispose(mouseDragMatchElem(this.scrollPane, '.field:not(.column_name)', (event, elem) => {
+    if (!ignoreEvent(event, elem)) {
+      // TODO: should always enable
+      if (!this.cellSelector.isSelected(elem, selector.CELL)) {
+        this.cellMouseDown(elem, event);
+        return {
+          onMove: (ev) => this.cellMouseMove(ev),
+          onStop: (ev) => {},
+        }
+      } else { // TODO: if true above, this will never come into play.
+        this.scheduleAssignCursor(elem, selector.NONE);
+        return {
+          onMove: (ev) => {},
+          onStop: (ev) => { this.cellSelector.currentDragType(selector.NONE); },
+        };
+      }
+    }
+  }));
+}
 
 // End of Selector stuff
 
@@ -1487,7 +1498,7 @@ GridView.prototype.styleColDragElements = function(elem, event) {
  * 3) If the last col/row is in the select range, the indicator line should be clamped to the start of the
  *    select range.
  **/
-GridView.prototype.dragRows = function(elem, event) {
+GridView.prototype.dragRows = function(event) {
   var dropIndex = Math.min(this.getMousePosRow(event.pageY + this.scrollTop()),
                            this.getLastDataRowIndex());
   if (this.cellSelector.containsRow(dropIndex)) {
@@ -1506,7 +1517,7 @@ GridView.prototype.dragRows = function(elem, event) {
   this.dragY(event.pageY);
 };
 
-GridView.prototype.dragCols = function(elem, event) {
+GridView.prototype.dragCols = function(event) {
   let dropIndex = Math.min(this.getMousePosCol(event.pageX),
                            this.viewSection.viewFields().peekLength - 1);
   if (this.cellSelector.containsCol(dropIndex)) {
@@ -1539,6 +1550,7 @@ GridView.prototype.dragCols = function(elem, event) {
 GridView.prototype.dropRows = function() {
   var oldIndices = _.range(this.cellSelector.rowLower(), this.cellSelector.rowUpper() + 1);
   this.moveRows(oldIndices, this.cellSelector.row.dropIndex());
+  this.cellSelector.currentDragType(selector.NONE);
 };
 
 GridView.prototype.dropCols = function() {
@@ -1552,6 +1564,7 @@ GridView.prototype.dropCols = function() {
     this.currentEditingColumnIndex(idx);
   }
   this._colClickTime = 0;
+  this.cellSelector.currentDragType(selector.NONE);
 };
 
 // End of Dragging logic
diff --git a/app/client/components/GristDoc.ts b/app/client/components/GristDoc.ts
index dc8e0509..4e26d685 100644
--- a/app/client/components/GristDoc.ts
+++ b/app/client/components/GristDoc.ts
@@ -1262,7 +1262,7 @@ const cssViewContentPane = styled('div', `
   flex: auto;
   display: flex;
   flex-direction: column;
-  overflow: hidden;
+  overflow: visible;
   position: relative;
   min-width: 240px;
   margin: 12px;
diff --git a/app/client/components/RawDataPage.ts b/app/client/components/RawDataPage.ts
index 476234ab..8e2bfa30 100644
--- a/app/client/components/RawDataPage.ts
+++ b/app/client/components/RawDataPage.ts
@@ -116,7 +116,7 @@ const cssOverlay = styled('div', `
   inset: 0px;
   height: 100%;
   width: 100%;
-  padding: 32px 56px 0px 56px;
+  padding: 20px 56px 20px 56px;
   position: absolute;
   @media ${mediaSmall} {
     & {
@@ -131,7 +131,6 @@ const cssSectionWrapper = styled('div', `
   height: 100%;
   display: flex;
   flex-direction: column;
-  overflow: hidden;
   border-radius: 5px;
   border-bottom-left-radius: 0px;
   border-bottom-right-radius: 0px;
diff --git a/app/client/components/SelectionSummary.ts b/app/client/components/SelectionSummary.ts
new file mode 100644
index 00000000..28b7265e
--- /dev/null
+++ b/app/client/components/SelectionSummary.ts
@@ -0,0 +1,339 @@
+import {CellSelector, COL, ROW} from 'app/client/components/CellSelector';
+import {copyToClipboard} from 'app/client/lib/copyToClipboard';
+import {Delay} from "app/client/lib/Delay";
+import {KoArray} from 'app/client/lib/koArray';
+import {ViewFieldRec} from 'app/client/models/entities/ViewFieldRec';
+import {UserError} from 'app/client/models/errors';
+import {ALL, RowsChanged, SortedRowSet} from "app/client/models/rowset";
+import {showTransientTooltip} from 'app/client/ui/tooltips';
+import {colors, isNarrowScreen, isNarrowScreenObs, theme, vars} from 'app/client/ui2018/cssVars';
+import {icon} from 'app/client/ui2018/icons';
+import {CellValue} from 'app/common/DocActions';
+import {isEmptyList, isListType, isRefListType} from "app/common/gristTypes";
+import {TableData} from "app/common/TableData";
+import {BaseFormatter} from 'app/common/ValueFormatter';
+import ko from 'knockout';
+import {Computed, Disposable, dom, makeTestId, Observable, styled, subscribe} from 'grainjs';
+
+/**
+ * A beginning and end index for a range of columns or rows.
+ */
+interface Range {
+  begin: number;
+  end: number;
+}
+
+/**
+ * A single part of the cell selection summary.
+ */
+interface SummaryPart {
+  /** Identifier for the summary part. */
+  id: 'sum' | 'count' | 'dimensions';
+  /** Label that's shown to the left of `value`. */
+  label: string;
+  /** Value of the summary part. */
+  value: string;
+  /** If true, displays a copy button on hover. Defaults to false. */
+  clickToCopy?: boolean;
+}
+
+const testId = makeTestId('test-selection-summary-');
+
+// We can handle a million cells in under 60ms on a good laptop. Much beyond that, and we'll break
+// selection with the bad performance. Instead, skip the counting and summing for too many cells.
+const MAX_CELLS_TO_SCAN = 1_000_000;
+
+export class SelectionSummary extends Disposable {
+  private _colTotalCount = Computed.create(this, (use) =>
+    use(use(this._viewFields).getObservable()).length);
+
+  private _rowTotalCount = Computed.create(this, (use) => {
+    const rowIds = use(this._sortedRows.getKoArray().getObservable());
+    const includesNewRow = (rowIds.length > 0 && rowIds[rowIds.length - 1] === 'new');
+    return rowIds.length - (includesNewRow ? 1 : 0);
+  });
+
+  // In CellSelector, start and end are 0-based, inclusive, and not necessarily in order.
+  // It's not good for representing an empty range. Here, we convert ranges as [begin, end),
+  // with end >= begin.
+  private _rowRange = Computed.create<Range>(this, (use) => {
+    const type = use(this._cellSelector.currentSelectType);
+    if (type === COL) {
+      return {begin: 0, end: use(this._rowTotalCount)};
+    } else {
+      const start = use(this._cellSelector.row.start);
+      const end = use(this._cellSelector.row.end);
+      return {
+        begin: Math.min(start, end),
+        end: Math.max(start, end) + 1,
+      };
+    }
+  });
+
+  private _colRange = Computed.create<Range>(this, (use) => {
+    const type = use(this._cellSelector.currentSelectType);
+    if (type === ROW) {
+      return {begin: 0, end: use(this._colTotalCount)};
+    } else {
+      const start = use(this._cellSelector.col.start);
+      const end = use(this._cellSelector.col.end);
+      return {
+        begin: Math.min(start, end),
+        end: Math.max(start, end) + 1,
+      };
+    }
+  });
+
+  private _summary = Observable.create<SummaryPart[]>(this, []);
+  private _delayedRecalc = this.autoDispose(Delay.create());
+
+  constructor(
+    private _cellSelector: CellSelector,
+    private _tableData: TableData,
+    private _sortedRows: SortedRowSet,
+    private _viewFields: ko.Computed<KoArray<ViewFieldRec>>,
+  ) {
+    super();
+
+    this.autoDispose(this._sortedRows.getKoArray().subscribe(this._onSpliceChange, this, 'spliceChange'));
+    const onRowNotify = this._onRowNotify.bind(this);
+    this._sortedRows.on('rowNotify', onRowNotify);
+    this.onDispose(() => this._sortedRows.off('rowNotify', onRowNotify));
+    this.autoDispose(subscribe(this._rowRange, this._colRange,
+      () => this._scheduleRecalc()));
+    this.autoDispose(isNarrowScreenObs().addListener((isNarrow) => {
+      if (isNarrow) { return; }
+      // No calculations occur while the screen is narrow, so we need to schedule one.
+      this._scheduleRecalc();
+    }));
+  }
+
+  public buildDom() {
+    return cssSummary(
+      dom.forEach(this._summary, ({id, label, value, clickToCopy}) =>
+        cssSummaryPart(
+          label ? dom('span', cssLabelText(label), cssCopyIcon('Copy')) : null,
+          value,
+          cssSummaryPart.cls('-copyable', Boolean(clickToCopy)),
+          (clickToCopy ? dom.on('click', (ev, elem) => doCopy(value, elem)) : null),
+          testId(id),
+        )
+      ),
+    );
+  }
+
+  private _onSpliceChange(splice: {start: number}) {
+    const rowRange = this._rowRange.get();
+    const rowCount = rowRange.end - rowRange.begin;
+    if (rowCount === 1) { return; }
+    if (splice.start >= rowRange.end) { return; }
+    // We could be smart here and only recalculate when the splice affects our selection. But for
+    // that to make sense, the selection itself needs to be smart. Currently, the selection is
+    // lost whenever the cursor is affected. For example, when you have a selection and another
+    // user adds/removes columns or rows before the selection, the selection won't be shifted
+    // with the cursor, and will instead be cleared. Since we can't always rely on the selection
+    // being there, we'll err on the safe side and always schedule a recalc.
+    this._scheduleRecalc();
+  }
+
+  private _onRowNotify(rows: RowsChanged) {
+    const rowRange = this._rowRange.get();
+    if (rows === ALL) {
+      this._scheduleRecalc();
+    } else {
+      const rowArray = this._sortedRows.getKoArray().peek();
+      const rowIdSet = new Set(rows);
+      for (let r = rowRange.begin; r < rowRange.end; r++) {
+        if (rowIdSet.has(rowArray[r])) {
+          this._scheduleRecalc();
+          break;
+        }
+      }
+    }
+  }
+
+  /**
+   * Schedules a re-calculation to occur in the immediate future.
+   *
+   * May be called repeatedly, but only a single re-calculation will be scheduled, to
+   * avoid queueing unnecessary amounts of work.
+   */
+  private _scheduleRecalc() {
+    // `_recalc` may take a non-trivial amount of time, so we defer until the stack is clear.
+    this._delayedRecalc.schedule(0, () => this._recalc());
+  }
+
+  private _recalc() {
+    const rowRange = this._rowRange.get();
+    const colRange = this._colRange.get();
+    let rowCount = rowRange.end - rowRange.begin;
+    let colCount = colRange.end - colRange.begin;
+    const cellCount = rowCount * colCount;
+    const summary: SummaryPart[] = [];
+    // Do nothing on narrow screens, because we haven't come up with a place to render sum anyway.
+    if (cellCount > 1 && !isNarrowScreen()) {
+      if (cellCount <= MAX_CELLS_TO_SCAN) {
+        const rowArray = this._sortedRows.getKoArray().peek();
+        const fields = this._viewFields.peek().peek();
+        let countNumeric = 0;
+        let countNonEmpty = 0;
+        let sum = 0;
+        let sumFormatter: BaseFormatter|null = null;
+        const rowIndices: number[] = [];
+        for (let r = rowRange.begin; r < rowRange.end; r++) {
+          const rowId = rowArray[r];
+          if (rowId === undefined || rowId === 'new') {
+            // We can run into this whenever the selection gets out of sync due to external
+            // changes, like another user removing some rows. For now, we'll skip rows that are
+            // still selected and no longer exist, but the real TODO is to better update the
+            // selection so that it doesn't have out-of-date and invalid ranges.
+            rowCount -= 1;
+            continue;
+          }
+          rowIndices.push(this._tableData.getRowIdIndex(rowId)!);
+        }
+        for (let c = colRange.begin; c < colRange.end; c++) {
+          const field = fields[c];
+          if (field === undefined) {
+            // Like with rows (see comment above), we need to watch out for out-of-date ranges.
+            colCount -= 1;
+            continue;
+          }
+          const col = fields[c].column.peek();
+          const displayCol = fields[c].displayColModel.peek();
+          const colType = col.type.peek();
+          const visibleColType = fields[c].visibleColModel.peek().type.peek();
+          const effectiveColType = visibleColType ?? colType;
+          const displayColId = displayCol.colId.peek();
+          // Note: we get values from the display column so that reference columns displaying
+          // numbers are included in the computed sum. Unfortunately, that also means we can't
+          // show a count of non-empty references. For now, that's a trade-off we'll have to make,
+          // but in the future it should be possible to allow showing multiple summary parts with
+          // some level of configurability.
+          const values = this._tableData.getColValues(displayColId);
+          if (!values) {
+            throw new UserError(`Invalid column ${this._tableData.tableId}.${displayColId}`);
+          }
+          const isNumeric = ['Numeric', 'Int', 'Any'].includes(effectiveColType);
+          const isEmpty: undefined | ((value: CellValue) => boolean) = (
+            colType.startsWith('Ref:') && !visibleColType ? value => (value === 0) :
+            isRefListType(colType) || isListType(effectiveColType) ? isEmptyList :
+            undefined
+          );
+          // The loops below are optimized, minimizing the amount of work done per row. For
+          // example, column values are retrieved in bulk above instead of once per row. In one
+          // unscientific test, they take 30-60ms per million numeric cells.
+          //
+          // TODO: Add a benchmark test suite that automates checking for performance regressions.
+          if (isNumeric) {
+            if (!sumFormatter) {
+              sumFormatter = fields[c].formatter.peek();
+            }
+            for (const i of rowIndices) {
+              const value = values[i];
+              if (typeof value === 'number') {
+                countNumeric++;
+                sum += value;
+              } else if (value !== null && value !== undefined && value !== '' && !isEmpty?.(value)) {
+                countNonEmpty++;
+              }
+            }
+          } else {
+            for (const i of rowIndices) {
+              const value = values[i];
+              if (value !== null && value !== undefined && value !== '' && !isEmpty?.(value)) {
+                countNonEmpty++;
+              }
+            }
+          }
+        }
+
+        if (countNumeric > 0) {
+          const sumValue = sumFormatter ? sumFormatter.formatAny(sum) : String(sum);
+          summary.push({id: 'sum', label: 'Sum ', value: sumValue, clickToCopy: true});
+        } else {
+          summary.push({id: 'count', label: 'Count ', value: String(countNonEmpty), clickToCopy: true});
+        }
+      }
+      summary.push({id: 'dimensions', label: '', value: `${rowCount}⨯${colCount}`});
+    }
+    this._summary.set(summary);
+  }
+}
+
+async function doCopy(value: string, elem: Element) {
+  await copyToClipboard(value);
+  showTransientTooltip(elem, 'Copied to clipboard', {key: 'copy-selection-summary'});
+}
+
+const cssSummary = styled('div', `
+  position: absolute;
+  bottom: -18px;
+  height: 18px;
+  line-height: 18px;
+  display: flex;
+  column-gap: 8px;
+  width: 100%;
+  justify-content: end;
+  color: ${theme.text};
+  /* Small hack: override the backdrop when viewing raw data to improve visibility. */
+  background-color: ${theme.mainPanelBg};
+  font-family: ${vars.fontFamilyData};
+
+  @media print {
+    & {
+      display: none;
+    }
+  }
+`);
+
+// Note: the use of an extra element for the background is to set its opacity, to make it a bit
+// lighter (or darker, in dark-mode) than actual mediumGrey, without defining a special color.
+const cssSummaryPart = styled('div', `
+  padding: 0 8px;
+  border-radius: 4px;
+  border-top-left-radius: 0px;
+  border-top-right-radius: 0px;
+  border-top: none;
+  z-index: 100;
+  position: relative;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+
+  &-copyable:hover {
+    cursor: pointer;
+  }
+  &::before {
+    content: "";
+    position: absolute;
+    top: 0;
+    left: 0;
+    bottom: 0;
+    right: 0;
+    background-color: ${colors.mediumGrey};
+    opacity: 0.8;
+    z-index: -1;
+  }
+`);
+
+const cssLabelText = styled('span', `
+  font-size: ${vars.xsmallFontSize};
+  text-transform: uppercase;
+  position: relative;
+  margin-right: 4px;
+  .${cssSummaryPart.className}-copyable:hover & {
+    visibility: hidden;
+  }
+`);
+
+const cssCopyIcon = styled(icon, `
+  position: absolute;
+  top: 0;
+  margin: 1px 0 0 4px;
+  --icon-color: ${theme.controlFg};
+  display: none;
+  .${cssSummaryPart.className}-copyable:hover & {
+    display: block;
+  }
+`);
diff --git a/app/client/components/Selector.js b/app/client/components/Selector.js
deleted file mode 100644
index 96471bc6..00000000
--- a/app/client/components/Selector.js
+++ /dev/null
@@ -1,291 +0,0 @@
-/**
- * Selector takes care of attaching callbacks to the relevant mouse events on the given view.
- * Selection and dragging/dropping consists of 3 phases: mouse down -> mouse move -> mouse up
- * The Selector class is purposefully lightweight because different views might have
- * different select/drag/drop behavior. Most of the work is done in the callbacks
- * provided to the Selector class.
- *
- * Usage:
-      Selectors are instantiated with a view.
-      @param{view}: The view containing the selectable/draggable elements
- *    Views must also supply the Selector class with mousedown/mousemove/mouseup callbacks and
- *    the associated element's that listen for the mouse events.
- *    through registerMouseHandlers.
- */
-
-/* globals document */
-
-var ko = require('knockout');
-var _ = require('underscore');
-var dispose = require('../lib/dispose');
-var gutil = require('app/common/gutil');
-const {isWin} = require('app/client/lib/browserInfo')
-
-var ROW = 'row';
-var COL = 'col';
-var CELL = 'cell';
-var NONE = '';
-var SELECT = 'select';
-var DRAG = 'drag';
-
-exports.ROW = ROW;
-exports.COL = COL;
-exports.CELL = CELL;
-exports.NONE = NONE;
-
-/**
- * @param {Object} view
- * @param {Object} opt
- * @param {function} opt.isDisabled - Is this selector disabled? Allows caller to specify
- *  conditions for temporarily disabling capturing of mouse events.
- */
-function Selector(view, opt) {
-  this.view = view;
-  // TODO: There should be a better way to ensure that select/drag doesnt happen when clicking
-  // on these things. Also, these classes should not be in the generic Selector class.
-  // TODO: get rid of the Selector class entirely and make this a Cell/GridSelector class specifically
-  // for GridView(and its derived views).
-  this.exemptClasses = [
-    'glyphicon-pencil',
-    'ui-resizable-handle',
-    'dropdown-toggle',
-  ];
-
-  opt = opt || {};
-
-  this.isDisabled = opt.isDisabled || _.constant(false);
-  this.isWindows = isWin();
-}
-
-/**
- * Register mouse callbacks to various sources.
- * @param {Object} callbacks -  an object containing mousedown/mouseup/mouseup functions
- * for selecting and dragging, along with with the source string name and target element
- * string name to which the mouse events must listen on.
- * @param {string} handlerName - string name of the kind of element that the mouse callbacks
- *                               are acting on.
- * handlerName is used to deduce what kind of element is triggering the mouse callbacks
- * The alternative is to look at triggering DOM element's css classes which is more hacky.
- */
-Selector.prototype.registerMouseHandlers = function(callbacks, handlerName) {
-  this.setCallbackDefaults(callbacks);
-  var self = this;
-
-  this.view.onEvent(callbacks.mousedown.source, 'mousedown', callbacks.mousedown.elemName,
-                    function(elem, event) {
-    if (self.isExemptMouseTarget(event) || event.button !== 0 || self.isDisabled()) {
-      return true; // Do nothing if the mouse event if exempt or not a left click
-    }
-
-    if (!self.isSelected(elem, handlerName) && !callbacks.disableSelect()) {
-      self.applyCallbacks(SELECT, callbacks, elem, event);
-    } else if (!callbacks.disableDrag()) {
-      self.applyCallbacks(DRAG, callbacks, elem, event);
-    }
-  });
-
-};
-
-Selector.prototype.isExemptMouseTarget = function(event) {
-  var cl = event.target.classList;
-  return _.some(this.exemptClasses, cl.contains.bind(cl));
-};
-
-Selector.prototype.setCallbackDefaults = function(callbacks) {
-  _.defaults(callbacks, {'mousedown': {}, 'mousemove': {}, 'mouseup': {},
-                         'disableDrag': _.constant(false), 'disableSelect': _.constant(false)}
-  );
-  _.defaults(callbacks.mousedown, {'select': _.noop, 'drag': _.noop, 'elemName': null,
-                                   'source': null});
-  _.defaults(callbacks.mousemove, {'select': _.noop, 'drag': _.noop, 'elemName': null,
-                                   'source': document});
-  _.defaults(callbacks.mouseup, {'select': _.noop, 'drag': _.noop, 'elemName': null,
-                                 'source': document});
-};
-
-/**
- * Applies the drag or select callback for mousedown and then registers
- * the appropriate mousemove and mouseup callbacks. We only register mousemove/mouseup
- * after seeing a mousedown event so that we don't have to constantly listen for
- * mousemove/mouseup.
- * @param {String} dragOrSelect - string that is either 'drag' or 'select' which denotes
- * which mouse methods to apply on mouse events.
- * @param {Object} callbacks -  an object containing mousedown/mouseup/mouseup functions
- * for selecting and dragging, along with with the source string name and target element
- * string name to which the mouse events must listen on.
- */
-Selector.prototype.applyCallbacks = function(dragOrSelect, callbacks, mouseDownElem, mouseDownEvent) {
-  console.assert(dragOrSelect === DRAG || dragOrSelect === SELECT);
-  var self = this;
-
-  callbacks.mousedown[dragOrSelect].call(this.view, mouseDownElem, mouseDownEvent);
-  this.view.onEvent(callbacks.mousemove.source, 'mousemove', function(elem, event) {
-    // On windows, when browser doesn't have focus, the first click produces artificial mousemove
-    // event. Fortunately, the mousemove event has the same coordinates as the mousedown event, so
-    // we will ignore it.
-    // Related issues:
-    // https://bugs.chromium.org/p/chromium/issues/detail?id=161464
-    // https://bugs.chromium.org/p/chromium/issues/detail?id=721341#c34
-    if (self.isWindows) {
-      if (event.screenX === mouseDownEvent.screenX && event.screenY === mouseDownEvent.screenY) {
-        return;
-      }
-    }
-    callbacks.mousemove[dragOrSelect].call(self.view, elem, event);
-  });
-
-  this.view.onEvent(callbacks.mouseup.source, 'mouseup', function(elem, event) {
-    callbacks.mouseup[dragOrSelect].call(self.view, elem, event);
-    self.view.clearEvent(callbacks.mousemove.source, 'mousemove');
-    self.view.clearEvent(callbacks.mouseup.source, 'mouseup');
-    if (dragOrSelect === DRAG) self.currentDragType(NONE);
-  });
-};
-
-// ===========================================================================
-// CELL SELECTOR
-
-function CellSelector(view, opt) {
-  Selector.call(this, view, opt);
-
-  // row or col.start denotes the anchor/initial index of the select range.
-  // start is not necessarily smaller than end.
-  // IE: clicking on col 10 and dragging until the mouse is on col 5 will yield: start = 10, end = 5
-  this.row = {
-    start: ko.observable(0),
-    end: ko.observable(0),
-    linePos: ko.observable('0px'),
-    dropIndex: ko.observable(-1),
-  };
-  this.col =  {
-    start: ko.observable(0),
-    end: ko.observable(0),
-    linePos: ko.observable('0px'),
-    dropIndex: ko.observable(-1),
-  };
-  this.currentSelectType = ko.observable(NONE);
-  this.currentDragType = ko.observable(NONE);
-
-  this.autoDispose(this.view.cursor.rowIndex.subscribeInit(function(rowIndex) {
-    this.setToCursor();
-  }, this));
-  this.autoDispose(this.view.cursor.fieldIndex.subscribeInit(function(colIndex) {
-    this.setToCursor();
-  }, this));
-}
-
-dispose.makeDisposable(CellSelector);
-_.extend(CellSelector.prototype, Selector.prototype);
-
-CellSelector.prototype.setToCursor = function(elemType) {
-  // Must check that the view contains cursor.rowIndex/cursor.fieldIndex
-  // in case it has changed.
-  if (this.view.cursor.rowIndex) {
-    this.row.start(this.view.cursor.rowIndex());
-    this.row.end(this.view.cursor.rowIndex());
-  }
-  if (this.view.cursor.fieldIndex) {
-    this.col.start(this.view.cursor.fieldIndex());
-    this.col.end(this.view.cursor.fieldIndex());
-  }
-  this.currentSelectType(elemType || NONE);
-};
-
-CellSelector.prototype.containsCell = function(rowIndex, colIndex) {
-  return this.containsCol(colIndex) && this.containsRow(rowIndex);
-};
-
-CellSelector.prototype.containsRow = function(rowIndex) {
-  return gutil.between(rowIndex, this.row.start(), this.row.end());
-};
-
-CellSelector.prototype.containsCol = function(colIndex) {
-  return gutil.between(colIndex, this.col.start(), this.col.end());
-};
-
-CellSelector.prototype.isSelected = function(elem, handlerName) {
-  if (handlerName !== this.currentSelectType()) return false;
-
-  // TODO: this only works with view: GridView.
-  // But it seems like we only ever use selectors with gridview anyway
-  let row = this.view.domToRowModel(elem, handlerName);
-  let col = this.view.domToColModel(elem, handlerName);
-  switch (handlerName) {
-    case ROW:
-      return this.containsRow(row._index());
-    case COL:
-      return this.containsCol(col._index());
-    case CELL:
-      return this.containsCell(row._index(), col._index());
-    default:
-      console.error('Given element is not a row, cell or column');
-      return false;
-  }
-};
-
-CellSelector.prototype.isRowSelected = function(rowIndex) {
-  return this.isCurrentSelectType(COL) || this.containsRow(rowIndex);
-};
-
-CellSelector.prototype.isColSelected = function(colIndex) {
-  return this.isCurrentSelectType(ROW) || this.containsCol(colIndex);
-};
-
-CellSelector.prototype.isCellSelected = function(rowIndex, colIndex) {
-  return this.isColSelected(colIndex) && this.isRowSelected(rowIndex);
-};
-
-CellSelector.prototype.onlyCellSelected = function(rowIndex, colIndex) {
-  return (this.row.start() === rowIndex && this.row.end() === rowIndex) &&
-         (this.col.start() === colIndex && this.col.end() === colIndex);
-};
-
-CellSelector.prototype.isCurrentSelectType = function(elemType) {
-  return this._isCurrentType(this.currentSelectType(), elemType);
-};
-
-CellSelector.prototype.isCurrentDragType = function(elemType) {
-  return this._isCurrentType(this.currentDragType(), elemType);
-};
-
-CellSelector.prototype._isCurrentType = function(currentType, elemType) {
-  console.assert([ROW, COL, CELL, NONE].indexOf(elemType) !== -1);
-  return currentType === elemType;
-};
-
-CellSelector.prototype.colLower = function() {
-  return Math.min(this.col.start(), this.col.end());
-};
-
-CellSelector.prototype.colUpper = function() {
-  return Math.max(this.col.start(), this.col.end());
-};
-
-CellSelector.prototype.rowLower = function() {
-  return Math.min(this.row.start(), this.row.end());
-};
-
-CellSelector.prototype.rowUpper = function() {
-  return Math.max(this.row.start(), this.row.end());
-};
-
-CellSelector.prototype.colCount = function() {
-  return this.colUpper() - this.colLower() + 1;
-};
-
-CellSelector.prototype.rowCount = function() {
-  return this.rowUpper() - this.rowLower() + 1;
-};
-
-CellSelector.prototype.selectArea = function(rowStartIdx, colStartIdx, rowEndIdx, colEndIdx) {
-  this.row.start(rowStartIdx);
-  this.col.start(colStartIdx);
-  this.row.end(rowEndIdx);
-  this.col.end(colEndIdx);
-  // Only select the area if it's not a single cell
-  if (this.colCount() > 1 || this.rowCount() > 1) {
-    this.currentSelectType(CELL);
-  }
-};
-
-exports.CellSelector = CellSelector;
diff --git a/app/client/components/ViewLayout.ts b/app/client/components/ViewLayout.ts
index 3eec3dc9..4e543674 100644
--- a/app/client/components/ViewLayout.ts
+++ b/app/client/components/ViewLayout.ts
@@ -15,7 +15,7 @@ import {reportError} from 'app/client/models/errors';
 import {filterBar} from 'app/client/ui/FilterBar';
 import {viewSectionMenu} from 'app/client/ui/ViewSectionMenu';
 import {buildWidgetTitle} from 'app/client/ui/WidgetTitle';
-import {mediaSmall, testId, theme} from 'app/client/ui2018/cssVars';
+import {isNarrowScreenObs, mediaSmall, testId, theme} from 'app/client/ui2018/cssVars';
 import {icon} from 'app/client/ui2018/icons';
 import {DisposableWithEvents} from 'app/common/DisposableWithEvents';
 import {mod} from 'app/common/gutil';
@@ -309,7 +309,7 @@ export function buildViewSectionDom(options: {
      )),
     dom.maybe((use) => use(vs.activeFilterBar) || use(vs.isRaw) && use(vs.activeFilters).length,
       () => dom.create(filterBar, vs)),
-    dom.maybe<BaseView|null>(vs.viewInstance, (viewInstance) =>
+    dom.maybe<BaseView|null>(vs.viewInstance, (viewInstance) => [
       dom('div.view_data_pane_container.flexvbox',
         cssResizing.cls('', isResizing),
         dom.maybe(viewInstance.disableEditing, () =>
@@ -319,9 +319,10 @@ export function buildViewSectionDom(options: {
           dom('div.viewsection_truncated', 'Not all data is shown')
         ),
         dom.cls((use) => 'viewsection_type_' + use(vs.parentKey)),
-        viewInstance.viewPane
-      )
-    ),
+        viewInstance.viewPane,
+      ),
+      dom.maybe(use => !use(isNarrowScreenObs()), () => viewInstance.selectionSummary?.buildDom()),
+    ]),
     dom.on('mousedown', () => { viewModel?.activeSectionId(sectionRowId); }),
   );
 }
diff --git a/app/client/declarations.d.ts b/app/client/declarations.d.ts
index 8c7ac1a8..64321d55 100644
--- a/app/client/declarations.d.ts
+++ b/app/client/declarations.d.ts
@@ -32,6 +32,7 @@ declare module "app/client/components/BaseView" {
 
   import {Cursor, CursorPos} from 'app/client/components/Cursor';
   import {GristDoc} from 'app/client/components/GristDoc';
+  import {SelectionSummary} from 'app/client/components/SelectionSummary';
   import {Disposable} from 'app/client/lib/dispose';
   import BaseRowModel from "app/client/models/BaseRowModel";
   import {DataRowModel} from 'app/client/models/DataRowModel';
@@ -61,6 +62,7 @@ declare module "app/client/components/BaseView" {
     public disableEditing: ko.Computed<boolean>;
     public isTruncated: ko.Observable<boolean>;
     public tableModel: DataTableModel;
+    public selectionSummary?: SelectionSummary;
 
     constructor(gristDoc: GristDoc, viewSectionModel: any, options?: {addNewRow?: boolean, isPreview?: boolean});
     public setCursorPos(cursorPos: CursorPos): void;
diff --git a/app/client/ui/mouseDrag.ts b/app/client/ui/mouseDrag.ts
index 34a39404..455bf4d6 100644
--- a/app/client/ui/mouseDrag.ts
+++ b/app/client/ui/mouseDrag.ts
@@ -30,6 +30,12 @@ export function mouseDrag(onStart: MouseDragStart): DomElementMethod {
   return (elem) => { mouseDragElem(elem, onStart); };
 }
 
+// Same as mouseDragElem, but listens for mousedown on descendants of elem that match selector.
+export function mouseDragMatchElem(elem: HTMLElement, selector: string, onStart: MouseDragStart): IDisposable {
+  return dom.onMatchElem(elem, selector, 'mousedown',
+    (ev, el) => _startDragging(ev as MouseEvent, el as HTMLElement, onStart));
+}
+
 function _startDragging(startEv: MouseEvent, elem: HTMLElement, onStart: MouseDragStart) {
   const dragHandler = onStart(startEv, elem);
   if (dragHandler) {
diff --git a/app/common/TableData.ts b/app/common/TableData.ts
index 73c4f65e..d4b9c614 100644
--- a/app/common/TableData.ts
+++ b/app/common/TableData.ts
@@ -169,18 +169,24 @@ export class TableData extends ActionDispatcher implements SkippableRows {
     return this._rowMap.has(rowId);
   }
 
+  /**
+   * Returns the index of the given rowId, if it exists, in the same unstable order that's
+   * returned by getRowIds() and getColValues().
+   */
+  public getRowIdIndex(rowId: UIRowId): number|undefined {
+    return this._rowMap.get(rowId as number);
+  }
+
   /**
    * Given a column name, returns a function that takes a rowId and returns the value for that
    * column of that row. The returned function is faster than getValue() calls.
    */
   public getRowPropFunc(colId: string): UIRowFunc<CellValue|undefined> {
+    const colData = this._columns.get(colId);
+    if (!colData) { return () => undefined; }
+    const values = colData.values;
     const rowMap = this._rowMap;
-    return (rowId: UIRowId) => {
-      const colData = this._columns.get(colId);
-      if (!colData) { return undefined; }
-      const values = colData.values;
-      return values[rowMap.get(rowId as number)!];
-    };
+    return (rowId: UIRowId) => values[rowMap.get(rowId as number)!];
   }
 
   // By default, no rows are skippable, all are kept.
diff --git a/test/fixtures/docs/SelectionSummary.grist b/test/fixtures/docs/SelectionSummary.grist
new file mode 100644
index 00000000..9d3168dd
Binary files /dev/null and b/test/fixtures/docs/SelectionSummary.grist differ
diff --git a/test/nbrowser/SelectionSummary.ts b/test/nbrowser/SelectionSummary.ts
new file mode 100644
index 00000000..264f060b
--- /dev/null
+++ b/test/nbrowser/SelectionSummary.ts
@@ -0,0 +1,211 @@
+import {assert, driver, Key, WebElement} from 'mocha-webdriver';
+import * as gu from 'test/nbrowser/gristUtils';
+import {setupTestSuite} from 'test/nbrowser/testUtils';
+
+interface CellPosition {
+  /** 0-based column index. */
+  col: number;
+  /** 0-based row index. */
+  row: number;
+}
+
+interface SelectionSummary {
+  dimensions: string;
+  count: number | null;
+  sum: string | null;
+}
+
+describe('SelectionSummary', function () {
+  this.timeout(20000);
+  const cleanup = setupTestSuite();
+
+  before(async function() {
+    const session = await gu.session().personalSite.login();
+    await session.tempDoc(cleanup, 'SelectionSummary.grist');
+  });
+
+  async function assertSelectionSummary(summary: SelectionSummary | null) {
+    if (!summary) {
+      assert.isFalse(await driver.find('.test-selection-summary-dimensions').isPresent());
+      assert.isFalse(await driver.find('.test-selection-summary-count').isPresent());
+      assert.isFalse(await driver.find('.test-selection-summary-sum').isPresent());
+      return;
+    }
+
+    const {dimensions, count, sum} = summary;
+    await gu.waitToPass(async () => assert.equal(
+      await driver.find('.test-selection-summary-dimensions').getText(),
+      dimensions
+    ), 500);
+    if (count === null) {
+      assert.isFalse(await driver.find('.test-selection-summary-count').isPresent());
+    } else {
+      await gu.waitToPass(async () => assert.equal(
+        await driver.find('.test-selection-summary-count').getText(),
+        `COUNT ${count}`
+      ), 500);
+    }
+    if (sum === null) {
+      assert.isFalse(await driver.find('.test-selection-summary-sum').isPresent());
+    } else {
+      await gu.waitToPass(async () => assert.equal(
+        await driver.find('.test-selection-summary-sum').getText(),
+        `SUM ${sum}`
+      ), 500);
+    }
+  }
+
+  function shiftClick(el: WebElement) {
+    return driver.withActions((actions) => actions.keyDown(Key.SHIFT).click(el).keyUp(Key.SHIFT));
+  }
+
+  async function selectAndAssert(start: CellPosition, end: CellPosition, summary: SelectionSummary | null) {
+    const {col: startCol, row: startRow} = start;
+    await gu.getCell(startCol, startRow + 1).click();
+    const {col: endCol, row: endRow} = end;
+    await shiftClick(await gu.getCell(endCol, endRow + 1));
+    await assertSelectionSummary(summary);
+  }
+
+  it('does not display anything if only a single cell is selected', async function () {
+    for (const [col, row] of [[0, 1], [2, 3]]) {
+      await gu.getCell(col, row).click();
+      await assertSelectionSummary(null);
+    }
+  });
+
+  it('displays sum if the selection contains numbers', async function () {
+    await selectAndAssert({col: 0, row: 0}, {col: 0, row: 6}, {
+      dimensions: '7⨯1',
+      count: null,
+      sum: '$135,692,590',
+    });
+    await selectAndAssert({col: 0, row: 3}, {col: 0, row: 6}, {
+      dimensions: '4⨯1',
+      count: null,
+      sum: '$135,679,011',
+    });
+
+    await selectAndAssert({col: 4, row: 0}, {col: 4, row: 6}, {
+      dimensions: '7⨯1',
+      count: null,
+      sum: '135692590',
+    });
+    await selectAndAssert({col: 0, row: 0}, {col: 4, row: 6}, {
+      dimensions: '7⨯5',
+      count: null,
+      sum: '$271,385,168.02',
+    });
+  });
+
+  it('uses formatter of the first (leftmost) numeric column', async function () {
+    // Column 0 is U.S. currency, while column 1 is just a plain decimal number.
+    await selectAndAssert({col: 0, row: 0}, {col: 1, row: 6}, {
+      dimensions: '7⨯2',
+      count: null,
+      sum: '$135,692,578.02',
+    });
+    await selectAndAssert({col: 1, row: 0}, {col: 1, row: 6}, {
+      dimensions: '7⨯1',
+      count: null,
+      sum: '-11.98',
+    });
+    // The entire selection (spanning 6 columns) uses the formatter of column 0.
+    await selectAndAssert({col: 0, row: 0}, {col: 5, row: 6}, {
+      dimensions: '7⨯6',
+      count: null,
+      sum: '$271,385,156.04',
+    });
+  });
+
+  it("displays count if the selection doesn't contain numbers", async function () {
+    await selectAndAssert({col: 2, row: 0}, {col: 2, row: 6}, {
+      dimensions: '7⨯1',
+      count: 5,
+      sum: null,
+    });
+    await selectAndAssert({col: 2, row: 3}, {col: 2, row: 6}, {
+      dimensions: '4⨯1',
+      count: 2,
+      sum: null,
+    });
+    await selectAndAssert({col: 2, row: 0}, {col: 3, row: 5}, {
+      dimensions: '6⨯2',
+      count: 11,
+      sum: null,
+    });
+
+    // Scroll horizontally to the end of the table.
+    await gu.sendKeys(Key.END);
+
+    await selectAndAssert({col: 7, row: 0}, {col: 10, row: 4}, {
+      dimensions: '5⨯4',
+      count: 7,
+      sum: null,
+    });
+    await selectAndAssert({col: 10, row: 0}, {col: 12, row: 6}, {
+      dimensions: '7⨯3',
+      count: 5,
+      sum: null,
+    });
+  });
+
+  it('uses the show column of reference columns for computations', async function () {
+    // Column 6 is a Reference column pointing to column 0.
+    await gu.sendKeys(Key.HOME);
+    await selectAndAssert({col: 6, row: 0}, {col: 6, row: 6}, {
+      dimensions: '7⨯1',
+      count: null,
+      sum: '-$123,456',
+    });
+
+    // Column 7 is a Reference List column pointing to column 0. At this time, it
+    // only displays counts (but flattening sums also seems like intuitive behavior).
+    await gu.sendKeys(Key.END);
+    await selectAndAssert({col: 7, row: 0}, {col: 7, row: 6}, {
+      dimensions: '7⨯1',
+      count: 2,
+      sum: null,
+    });
+  });
+
+  it('updates whenever the selection changes', async function () {
+    // Scroll horizontally to the beginning of the table.
+    await gu.sendKeys(Key.HOME);
+
+    // Select a region of the table.
+    await selectAndAssert({col: 0, row: 2}, {col: 0, row: 6}, {
+      dimensions: '5⨯1',
+      count: null,
+      sum: '$135,691,356',
+    });
+
+    // Without de-selecting, use keyboard shortcuts to grow the selection to the right.
+    await gu.sendKeys(Key.chord(Key.SHIFT, Key.ARROW_RIGHT));
+
+    // Check that the selection summary was updated.
+    await assertSelectionSummary({
+      dimensions: '5⨯2',
+      count: null,
+      sum: '$135,691,368.5',
+    });
+  });
+
+  it('displays correct sum when all rows/columns are selected', async function () {
+    await driver.find(".gridview_data_corner_overlay").click();
+    await assertSelectionSummary({
+      dimensions: '7⨯14',
+      count: null,
+      sum: '$271,261,700.04',
+    });
+  });
+
+  describe('on narrow screens', function() {
+    gu.narrowScreen();
+
+    it('is not visible', async function() {
+      await assertSelectionSummary(null);
+      await selectAndAssert({col: 0, row: 0}, {col: 0, row: 6}, null);
+    });
+  });
+ });