fix(editor): behavior of deleting at the start of line (#12787)

Close BS-3182, #12736 




#### PR Dependency Tree


* **PR #12787** 👈

This tree was auto-generated by
[Charcoal](https://github.com/danerwilliams/charcoal)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

- **Bug Fixes**
- Improved the behavior when deleting empty lines and merging blocks,
ensuring more accurate handling of block deletion and cursor focus in
various scenarios.
- **Tests**
- Added new end-to-end tests to verify correct deletion of lines in
edgeless text and paragraph blocks, including checks for block removal
and cursor position.
- Introduced a utility function to retrieve block IDs for testing
purposes.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

tests/blocksuite/e2e/paragraph.spec.ts

@@ -8,7 +8,9 @@ import {
   enterPlaygroundRoom,
   focusRichText,
   focusTitle,
+  getBlockIds,
   getIndexCoordinate,
+  getInlineSelectionIndex,
   getPageSnapshot,
   initEmptyEdgelessState,
   initEmptyParagraphState,
@@ -44,6 +46,7 @@ import {
   assertBlockChildrenIds,
   assertBlockCount,
   assertBlockSelections,
+  assertBlockTextContent,
   assertBlockType,
   assertClassName,
   assertDivider,
@@ -734,6 +737,45 @@ test('delete at start of paragraph with content', async ({ page }) => {
   await assertRichTexts(page, ['123', '456']);
 });
 
+test('delete empty line should work correctly', async ({ page }) => {
+  await enterPlaygroundRoom(page);
+  await initEmptyParagraphState(page);
+  await focusRichText(page);
+
+  // 2: aaa
+  // 3:     bbb
+  // 4: ccc|
+  {
+    await type(page, 'aaa');
+    await pressEnter(page);
+    await pressTab(page);
+    await type(page, 'bbb');
+    await pressEnter(page);
+    await pressShiftTab(page);
+    await type(page, 'ccc');
+  }
+
+  // 2: aaa
+  // 3:     bbb|
+  await pressBackspace(page, 4);
+  expect(await getBlockIds(page)).not.toContain(4);
+  await assertBlockTextContent(page, 2, 'aaa');
+  await assertBlockTextContent(page, 3, 'bbb');
+  expect(await getInlineSelectionIndex(page)).toBe(3);
+
+  // title: |aaa
+  // 3: bbb
+  {
+    await pressArrowUp(page);
+    await pressArrowLeft(page, 3);
+    await pressBackspace(page);
+  }
+  await expect(page.locator('doc-title')).toContainText('aaa');
+  expect(await getBlockIds(page)).not.toContain(2);
+  await assertBlockTextContent(page, 3, 'bbb');
+  expect(await getInlineSelectionIndex(page)).toBe(0);
+});
+
 test('get focus from page title enter', async ({ page }) => {
   await enterPlaygroundRoom(page);
   await initEmptyParagraphState(page);