Preserve native undo/redo in markdown toolbar buttons (#107)

insertMarkdownFormat, insertMarkdownText, toolbarList, toolbarHeading,
and toolbarQuote all set textarea.value = ... directly. Assigning
.value programmatically discards the browser's entire native undo
stack (vs. document.execCommand('insertText', ...), which preserves
it) — e.g. type a paragraph, click Bold, then Ctrl+Z undid the whole
paragraph instead of just the bold markup.

Added insertTextPreservingUndo(), which selects the exact range being
replaced and routes through execCommand('insertText', ...) — the same
mechanism real typing uses — falling back to the old direct assignment
(losing undo, matching prior behavior) only if execCommand is
unavailable or unsuccessful.

Verified with a jsdom harness that the fallback path (jsdom doesn't
implement execCommand, since native undo is a real-browser-only
feature untestable via jsdom) produces byte-identical resulting text
and cursor positions to the original implementation across all 5
toolbar functions, for both selected and cursor-only cases.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

assets/js/markdown.js

@@ -354,6 +354,33 @@ window.renderMarkdownElements = renderMarkdownElements;
 // Rich Text Editor Toolbar Functions
 // ========================================
 
+/**
+ * Replace textarea.value.substring(selStart, selEnd) with replacementText,
+ * preserving the browser's native undo/redo stack via
+ * document.execCommand('insertText', ...) -- the same mechanism real typing
+ * uses -- instead of a direct .value assignment, which discards the entire
+ * undo history. Falls back to a direct assignment (losing undo, matching the
+ * old behavior) only if execCommand is unavailable or unsuccessful.
+ */
+function insertTextPreservingUndo(textarea, replacementText, selStart, selEnd) {
+    textarea.focus();
+    textarea.setSelectionRange(selStart, selEnd);
+
+    let inserted = false;
+    if (typeof document.execCommand === 'function') {
+        try {
+            inserted = document.execCommand('insertText', false, replacementText);
+        } catch (e) {
+            inserted = false;
+        }
+    }
+
+    if (!inserted) {
+        const text = textarea.value;
+        textarea.value = text.substring(0, selStart) + replacementText + text.substring(selEnd);
+    }
+}
+
 /**
  * Insert markdown formatting around selection
  */
@@ -363,16 +390,13 @@ function insertMarkdownFormat(textareaId, prefix, suffix) {
 
     const start = textarea.selectionStart;
     const end = textarea.selectionEnd;
-    const text = textarea.value;
-    const selectedText = text.substring(start, end);
+    const selectedText = textarea.value.substring(start, end);
 
-    // Insert formatting
-    const newText = text.substring(0, start) + prefix + selectedText + suffix + text.substring(end);
-    textarea.value = newText;
+    insertTextPreservingUndo(textarea, prefix + selectedText + suffix, start, end);
 
     // Set cursor position
     if (selectedText) {
-        textarea.setSelectionRange(start + prefix.length, end + prefix.length);
+        textarea.setSelectionRange(start + prefix.length, start + prefix.length + selectedText.length);
     } else {
         textarea.setSelectionRange(start + prefix.length, start + prefix.length);
     }
@@ -391,9 +415,10 @@ function insertMarkdownText(textareaId, text) {
     if (!textarea) return;
 
     const start = textarea.selectionStart;
-    const value = textarea.value;
 
-    textarea.value = value.substring(0, start) + text + value.substring(start);
+    // Matches the prior behavior: insert before the selection start without
+    // deleting any currently-selected text (a collapsed replace range).
+    insertTextPreservingUndo(textarea, text, start, start);
     textarea.setSelectionRange(start + text.length, start + text.length);
     textarea.focus();
 
@@ -453,7 +478,7 @@ function toolbarList(textareaId) {
     }
 
     // Insert list marker at beginning of line
-    textarea.value = text.substring(0, lineStart) + '- ' + text.substring(lineStart);
+    insertTextPreservingUndo(textarea, '- ', lineStart, lineStart);
     textarea.setSelectionRange(start + 2, start + 2);
     textarea.focus();
 
@@ -474,7 +499,7 @@ function toolbarHeading(textareaId) {
     }
 
     // Insert heading marker at beginning of line
-    textarea.value = text.substring(0, lineStart) + '## ' + text.substring(lineStart);
+    insertTextPreservingUndo(textarea, '## ', lineStart, lineStart);
     textarea.setSelectionRange(start + 3, start + 3);
     textarea.focus();
 
@@ -495,7 +520,7 @@ function toolbarQuote(textareaId) {
     }
 
     // Insert quote marker at beginning of line
-    textarea.value = text.substring(0, lineStart) + '> ' + text.substring(lineStart);
+    insertTextPreservingUndo(textarea, '> ', lineStart, lineStart);
     textarea.setSelectionRange(start + 2, start + 2);
     textarea.focus();