fix(sheet): Delete clears a single selected cell (#364)

Delete/Backspace only cleared multi-cell ranges; a single selected cell
was left to native contenteditable, unlike Excel. Replace the single-cell
exclusion with a grid-focus guard: Delete now clears one cell too, while
Backspace still edits the formula bar (and any input outside the grid).

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: SamTV12345

playwright/specs/sheet_selection.spec.ts

@@ -135,4 +135,26 @@ test.describe('Sheet selection, fill, and clipboard', () => {
       await expect(cell(page, 2, 1)).toHaveText('bar', { timeout: 10000 });
     });
   });
+
+  test('Delete clears a single selected cell', async ({ page }) => {
+    const padId = `sheet-del1-${Date.now()}`;
+    await openSheet(page, padId);
+    await commitCell(page, 0, 0, 'hallo'); // A1
+    await cell(page, 0, 0).click();
+    await page.keyboard.press('Delete');
+    await expect(cell(page, 0, 0)).toHaveText('');
+  });
+
+  test('Backspace in the formula bar edits the formula, not clears the cell', async ({ page }) => {
+    const padId = `sheet-fxbksp-${Date.now()}`;
+    await openSheet(page, padId);
+    await commitCell(page, 0, 0, 'abc'); // A1
+    await cell(page, 0, 0).click();
+    const fx = page.locator('.sheet-fx-input');
+    await fx.click();
+    await page.keyboard.press('Backspace'); // must act on fx text, not wipe A1
+    await page.keyboard.press('Escape'); // cancel the edit
+    // If the document Delete handler had fired, A1 would be empty regardless.
+    await expect(cell(page, 0, 0)).toHaveText('abc');
+  });
 });

ui/src/js/sheet/sheetEditor.ts

@@ -573,7 +573,15 @@ export function startSheetEditor(root: HTMLElement): void {
       doPaste();
       return;
     }
-    if ((e.key === 'Delete' || e.key === 'Backspace') && !editingNow() && !readOnly && !selIsSingle(selection)) {
+    // Clear the selection (single cell or range), like Excel. The grid-focus
+    // guard replaces the old single-cell exclusion: it lets Delete clear one
+    // cell while still keeping Backspace working in the formula bar and any
+    // other input outside the grid (selection is single there too).
+    if (
+      (e.key === 'Delete' || e.key === 'Backspace') &&
+      !editingNow() && !readOnly &&
+      (e.target as HTMLElement | null)?.closest?.('.sheet-grid')
+    ) {
       e.preventDefault();
       blurActiveCell();
       const { r0, c0, r1, c1 } = normalize(selection);