(core) Fix linking of new records when attachment is the first thing added.

Summary:
Fixes a bug when in a linked widget, the automatic reference wasn't being set
for a new record if attachment is the first thing that gets added to the
record.

- Move handling of 'setCursorPos' pseudo-command to GristDoc to support cross-section switching (relevant when moving attachment into a cell of a non-active page widget)
- Modernize code for AttachmentsWidget slightly (better typings, css conventions)
- Change the fix in https://phab.getgrist.com/D3796 from using isolate to using different z-index values, to avoid a change in the look of the cursor on Attachment cells.

Test Plan: Added a test case for what's possible to test with webdriver.

Reviewers: georgegevoian

Reviewed By: georgegevoian

Differential Revision: https://phab.getgrist.com/D3811

test/nbrowser/AttachmentsLinking.ts

@@ -0,0 +1,80 @@
+import {assert} from 'mocha-webdriver';
+import * as gu from 'test/nbrowser/gristUtils';
+import {Session} from 'test/nbrowser/gristUtils';
+import {setupTestSuite} from 'test/nbrowser/testUtils';
+
+describe('AttachmentsLinking', function() {
+  this.timeout(20000);
+
+  const cleanup = setupTestSuite({team: true});
+  let session: Session;
+  let docId: string;
+
+  before(async function() {
+    session = await gu.session().login();
+    docId = await session.tempNewDoc(cleanup, 'AttachmentColumns', {load: false});
+
+    // Set up a table Src, and table Items which links to Src and has an Attachments column.
+    const api = session.createHomeApi();
+    await api.applyUserActions(docId, [
+      ['AddTable', 'Src', [{id: 'A', type: 'Text'}]],
+      ['BulkAddRecord', 'Src', [null, null, null], {A: ['a', 'b', 'c']}],
+      ['AddTable', 'Items', [
+        {id: 'A', type: 'Ref:Src'},
+        {id: 'Att', type: 'Attachments'},
+      ]],
+      ['BulkAddRecord', 'Items', [null, null, null], {A: [1, 1, 3]}],
+    ]);
+
+    await session.loadDoc(`/doc/${docId}`);
+
+    // Set up a page with linked widgets.
+    await gu.addNewPage('Table', 'Src');
+    await gu.addNewSection('Table', 'Items', {selectBy: /Src/i});
+  });
+
+  it('should fill in values determined by linking when uploading to the add row', async function() {
+    // TODO Another good test case would be that dragging a file into a cell works, especially
+    // when that cell isn't in the selected widget. But this doesn't seem supported by webdriver.
+
+    // Selecting a cell in Src should show only linked values in Items.
+    await gu.getCell({section: 'Src', col: 'A', rowNum: 1}).click();
+    assert.deepEqual(await gu.getVisibleGridCells({section: 'Items', cols: ['A', 'Att'], rowNums: [1, 2, 3]}), [
+      'Src[1]', '',
+      'Src[1]', '',
+      '', '',
+    ]);
+
+    // Upload into an Attachments cell in the "Add Row" of Items.
+    assert.equal(await gu.getCell({section: 'Items', col: 0, rowNum: 4}).isPresent(), false);
+
+    let cell = await gu.getCell({section: 'Items', col: 'Att', rowNum: 3});
+    await gu.fileDialogUpload('uploads/file1.mov', () => cell.find('.test-attachment-icon').click());
+    await gu.waitToPass(async () =>
+      assert.lengthOf(await gu.getCell({section: 'Items', col: 'Att', rowNum: 3}).findAll('.test-pw-thumbnail'), 1));
+
+    assert.deepEqual(await gu.getVisibleGridCells({section: 'Items', cols: ['A', 'Att'], rowNums: [1, 2, 3, 4]}), [
+      'Src[1]', '',
+      'Src[1]', '',
+      'Src[1]', 'MOV',
+      '', '',
+    ]);
+
+    // Switch to another Src row; should see no attachments.
+    await gu.getCell({section: 'Src', col: 'A', rowNum: 2}).click();
+    assert.deepEqual(await gu.getVisibleGridCells({section: 'Items', cols: ['A', 'Att'], rowNums: [1]}), [
+      '', '',
+    ]);
+
+    cell = await gu.getCell({section: 'Items', col: 'Att', rowNum: 1});
+    await gu.fileDialogUpload('uploads/htmlfile.html,uploads/file1.mov',
+      () => cell.find('.test-attachment-icon').click());
+    await gu.waitToPass(async () =>
+      assert.lengthOf(await gu.getCell({section: 'Items', col: 'Att', rowNum: 1}).findAll('.test-pw-thumbnail'), 2));
+
+    assert.deepEqual(await gu.getVisibleGridCells({section: 'Items', cols: ['A', 'Att'], rowNums: [1, 2]}), [
+      'Src[2]', 'HTML\nMOV',
+      '', '',
+    ]);
+  });
+});