Add matching pair insertion to markdown textarea (#36121)

1. Our textarea already has some editor-like feature like tab
indentation, so I thought why not also add insertion of matching closing
quotes/brackets over selected text. This does that.
2. `textareaInsertText` is replaced with `replaceTextareaSelection`
which does the same but create a new edit history entry in the textarea
so CTRL-Z works. The button that inserts tables into the textarea can
now also be reverted via CTRL-Z, which was not possible before.

web_src/js/features/comp/ComboMarkdownEditor.ts

@@ -17,7 +17,7 @@ import {POST} from '../../modules/fetch.ts';
 import {
   EventEditorContentChanged,
   initTextareaMarkdown,
-  textareaInsertText,
+  replaceTextareaSelection,
   triggerEditorContentChanged,
 } from './EditorMarkdown.ts';
 import {DropzoneCustomEventReloadFiles, initDropzone} from '../dropzone.ts';
@@ -273,7 +273,7 @@ export class ComboMarkdownEditor {
       let cols = parseInt(addTablePanel.querySelector<HTMLInputElement>('[name=cols]')!.value);
       rows = Math.max(1, Math.min(100, rows));
       cols = Math.max(1, Math.min(100, cols));
-      textareaInsertText(this.textarea, `\n${this.generateMarkdownTable(rows, cols)}\n\n`);
+      replaceTextareaSelection(this.textarea, `\n${this.generateMarkdownTable(rows, cols)}\n\n`);
       addTablePanelTippy.hide();
     });
   }

web_src/js/features/comp/EditorMarkdown.ts

@@ -4,14 +4,23 @@ export function triggerEditorContentChanged(target: HTMLElement) {
   target.dispatchEvent(new CustomEvent(EventEditorContentChanged, {bubbles: true}));
 }
 
-export function textareaInsertText(textarea: HTMLTextAreaElement, value: string) {
-  const startPos = textarea.selectionStart;
-  const endPos = textarea.selectionEnd;
-  textarea.value = textarea.value.substring(0, startPos) + value + textarea.value.substring(endPos);
-  textarea.selectionStart = startPos;
-  textarea.selectionEnd = startPos + value.length;
+/** replace selected text or insert text by creating a new edit history entry,
+ *  e.g. CTRL-Z works after this */
+export function replaceTextareaSelection(textarea: HTMLTextAreaElement, text: string) {
+  const before = textarea.value.slice(0, textarea.selectionStart);
+  const after = textarea.value.slice(textarea.selectionEnd);
+
   textarea.focus();
-  triggerEditorContentChanged(textarea);
+  let success = false;
+  try {
+    success = document.execCommand('insertText', false, text); // eslint-disable-line @typescript-eslint/no-deprecated
+  } catch {}
+
+  // fall back to regular replacement
+  if (!success) {
+    textarea.value = `${before}${text}${after}`;
+    triggerEditorContentChanged(textarea);
+  }
 }
 
 type TextareaValueSelection = {
@@ -176,7 +185,7 @@ export function markdownHandleIndention(tvs: TextareaValueSelection): MarkdownHa
   return {handled: true, valueSelection: {value: linesBuf.lines.join('\n'), selStart: newPos, selEnd: newPos}};
 }
 
-function handleNewline(textarea: HTMLTextAreaElement, e: Event) {
+function handleNewline(textarea: HTMLTextAreaElement, e: KeyboardEvent) {
   const ret = markdownHandleIndention({value: textarea.value, selStart: textarea.selectionStart, selEnd: textarea.selectionEnd});
   if (!ret.handled || !ret.valueSelection) return; // FIXME: the "handled" seems redundant, only valueSelection is enough (null for unhandled)
   e.preventDefault();
@@ -185,6 +194,28 @@ function handleNewline(textarea: HTMLTextAreaElement, e: Event) {
   triggerEditorContentChanged(textarea);
 }
 
+// Keys that act as dead keys will not work because the spec dictates that such keys are
+// emitted as `Dead` in e.key instead of the actual key.
+const pairs = new Map<string, string>([
+  ["'", "'"],
+  ['"', '"'],
+  ['`', '`'],
+  ['(', ')'],
+  ['[', ']'],
+  ['{', '}'],
+  ['<', '>'],
+]);
+
+function handlePairCharacter(textarea: HTMLTextAreaElement, e: KeyboardEvent): void {
+  const selStart = textarea.selectionStart;
+  const selEnd = textarea.selectionEnd;
+  if (selEnd === selStart) return; // do not process when no selection
+  e.preventDefault();
+  const inner = textarea.value.substring(selStart, selEnd);
+  replaceTextareaSelection(textarea, `${e.key}${inner}${pairs.get(e.key)}`);
+  textarea.setSelectionRange(selStart + 1, selEnd + 1);
+}
+
 function isTextExpanderShown(textarea: HTMLElement): boolean {
   return Boolean(textarea.closest('text-expander')?.querySelector('.suggestions'));
 }
@@ -198,6 +229,8 @@ export function initTextareaMarkdown(textarea: HTMLTextAreaElement) {
     } else if (e.key === 'Enter' && !e.shiftKey && !e.ctrlKey && !e.metaKey && !e.altKey) {
       // use Enter to insert a new line with the same indention and prefix
       handleNewline(textarea, e);
+    } else if (pairs.has(e.key)) {
+      handlePairCharacter(textarea, e);
     }
   });
 }

web_src/js/features/comp/EditorUpload.ts

@@ -1,5 +1,5 @@
 import {imageInfo} from '../../utils/image.ts';
-import {textareaInsertText, triggerEditorContentChanged} from './EditorMarkdown.ts';
+import {replaceTextareaSelection, triggerEditorContentChanged} from './EditorMarkdown.ts';
 import {
   DropzoneCustomEventRemovedFile,
   DropzoneCustomEventUploadDone,
@@ -43,7 +43,7 @@ class TextareaEditor {
   }
 
   insertPlaceholder(value: string) {
-    textareaInsertText(this.editor, value);
+    replaceTextareaSelection(this.editor, value);
   }
 
   replacePlaceholder(oldVal: string, newVal: string) {