(core) Selection for styled columns

Summary:
Grid UI improvements.
- Selecting a column with a custom background, didn't produce a visual difference.
- Hiding num-row right border when it is not needed (it should be used only for frozen columns)

Test Plan: manual tests

Reviewers: cyprien

Reviewed By: cyprien

Differential Revision: https://phab.getgrist.com/D3425

app/client/components/GridView.css

@@ -235,7 +235,6 @@
   width: 0px; /* Matches rowid width (+border) */
   height: 100%;
   z-index: 3;
-  left: calc(4rem);
   border-right: 1px solid var(--grist-color-dark-grey) !important;
   user-select: none;
   pointer-events: none

app/client/components/GridView.js

@@ -924,7 +924,13 @@ GridView.prototype.buildDom = function() {
     dom('div.frozen_line', kd.show(this.frozenLine)),
     dom('div.gridview_header_backdrop_left'), //these hide behind the actual headers to keep them from flashing
     dom('div.gridview_header_backdrop_top'),
-    dom('div.gridview_left_border'), //these hide behind the actual headers to keep them from flashing
+    // When there are frozen columns, right border for number row will not be visible (as actually there is no border,
+    // it comes from the first cell in the grid) making a gap between row-number and actual column. So when we scroll
+    // the content of the scrolled columns will be visible to the user (as there is blank space there).
+    // This line fills the gap. NOTE that we are using number here instead of a boolean.
+    dom('div.gridview_left_border', kd.show(this.numFrozen),
+      kd.style("left", ROW_NUMBER_WIDTH + 'px')
+    ),
     // left shadow that will be visible on top of frozen columns
     dom('div.scroll_shadow_frozen', kd.show(this.frozenShadow)),
     // When cursor leaves the GridView, remove hover immediately (without debounce).

app/client/components/viewCommon.css

@@ -75,6 +75,10 @@
   color: var(--grist-actual-cell-color, black);
 }
 
+.field.selected .field_clip {
+  mix-blend-mode: darken;
+}
+
 .field_clip.invalid, .field_clip.field-error-from-style {
   background-color: #ffb6c1;
   color: black;