fix textarea newline handle (#32966)

- Fix cursor position if input newline on middle of lines
- ~Increment number if numbered list~


![image](https://github.com/user-attachments/assets/bcfe2625-11a8-4ea4-9a71-b7ecfe81b2e0)

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

web_src/js/features/comp/EditorMarkdown.test.ts

@@ -4,13 +4,24 @@ test('EditorMarkdown', () => {
   const textarea = document.createElement('textarea');
   initTextareaMarkdown(textarea);
 
-  const testInput = (value, expected) => {
-    textarea.value = value;
-    textarea.setSelectionRange(value.length, value.length);
+  type ValueWithCursor = string | {
+    value: string;
+    pos: number;
+  }
+  const testInput = (input: ValueWithCursor, result: ValueWithCursor) => {
+    const intputValue = typeof input === 'string' ? input : input.value;
+    const inputPos = typeof input === 'string' ? intputValue.length : input.pos;
+    textarea.value = intputValue;
+    textarea.setSelectionRange(inputPos, inputPos);
+
     const e = new KeyboardEvent('keydown', {key: 'Enter', cancelable: true});
     textarea.dispatchEvent(e);
-    if (!e.defaultPrevented) textarea.value += '\n';
-    expect(textarea.value).toEqual(expected);
+    if (!e.defaultPrevented) textarea.value += '\n'; // simulate default behavior
+
+    const expectedValue = typeof result === 'string' ? result : result.value;
+    const expectedPos = typeof result === 'string' ? expectedValue.length : result.pos;
+    expect(textarea.value).toEqual(expectedValue);
+    expect(textarea.selectionStart).toEqual(expectedPos);
   };
 
   testInput('-', '-\n');
@@ -18,8 +29,11 @@ test('EditorMarkdown', () => {
 
   testInput('- ', '');
   testInput('1. ', '');
+  testInput({value: '1. \n2. ', pos: 3}, {value: '\n2. ', pos: 0});
 
   testInput('- x', '- x\n- ');
+  testInput('1. foo', '1. foo\n1. ');
+  testInput({value: '1. a\n2. b\n3. c', pos: 4}, {value: '1. a\n1. \n2. b\n3. c', pos: 8});
   testInput('- [ ]', '- [ ]\n- ');
   testInput('- [ ] foo', '- [ ] foo\n- [ ] ');
   testInput('* [x] foo', '* [x] foo\n* [ ] ');

web_src/js/features/comp/EditorMarkdown.ts

@@ -92,6 +92,7 @@ function handleNewline(textarea: HTMLTextAreaElement, e: Event) {
   if (!line) {
     // clear current line if we only have i.e. '1. ' and the user presses enter again to finish creating a list
     textarea.value = value.slice(0, lineStart) + value.slice(lineEnd);
+    textarea.setSelectionRange(selStart - prefix.length, selStart - prefix.length);
   } else {
     // start a new line with the same indention and prefix
     let newPrefix = prefix;