Replace textarea.value calls

assets/editor.js

@@ -6,44 +6,55 @@
 
     // --- DOM helpers ---
 
+    // Route every edit through execCommand so the browser's native undo/redo
+    // stack is preserved. Direct assignment to textarea.value would wipe it.
+    function replaceRange(start, end, text) {
+        textarea.focus();
+        textarea.selectionStart = start;
+        textarea.selectionEnd = end;
+        document.execCommand('insertText', false, text);
+    }
+
     function wrap(before, after, placeholder) {
         var start = textarea.selectionStart;
         var end = textarea.selectionEnd;
-        var selected = textarea.value.slice(start, end) || placeholder;
-        var replacement = before + selected + after;
-        textarea.value = textarea.value.slice(0, start) + replacement + textarea.value.slice(end);
-        if (selected === placeholder) {
+        var hadSelection = end > start;
+        var selected = hadSelection ? textarea.value.slice(start, end) : placeholder;
+        replaceRange(start, end, before + selected + after);
+        if (!hadSelection) {
             textarea.selectionStart = start + before.length;
             textarea.selectionEnd = start + before.length + placeholder.length;
-        } else {
-            textarea.selectionStart = start + replacement.length;
-            textarea.selectionEnd = start + replacement.length;
         }
-        textarea.focus();
     }
 
     function linePrefix(prefix) {
         var start = textarea.selectionStart;
         var lineStart = textarea.value.lastIndexOf('\n', start - 1) + 1;
-        textarea.value = textarea.value.slice(0, lineStart) + prefix + textarea.value.slice(lineStart);
+        replaceRange(lineStart, lineStart, prefix);
         textarea.selectionStart = textarea.selectionEnd = start + prefix.length;
-        textarea.focus();
     }
 
     function insertAtCursor(s) {
-        var start = textarea.selectionStart;
-        var end = textarea.selectionEnd;
-        textarea.value = textarea.value.slice(0, start) + s + textarea.value.slice(end);
-        textarea.selectionStart = textarea.selectionEnd = start + s.length;
-        textarea.dispatchEvent(new Event('input'));
-        textarea.focus();
+        replaceRange(textarea.selectionStart, textarea.selectionEnd, s);
     }
 
     function applyResult(result) {
-        textarea.value = result.text;
+        var oldText = textarea.value;
+        var newText = result.text;
+        var prefixLen = 0;
+        var maxPrefix = Math.min(oldText.length, newText.length);
+        while (prefixLen < maxPrefix && oldText.charCodeAt(prefixLen) === newText.charCodeAt(prefixLen)) {
+            prefixLen++;
+        }
+        var oldEnd = oldText.length;
+        var newEnd = newText.length;
+        while (oldEnd > prefixLen && newEnd > prefixLen
+                && oldText.charCodeAt(oldEnd - 1) === newText.charCodeAt(newEnd - 1)) {
+            oldEnd--;
+            newEnd--;
+        }
+        replaceRange(prefixLen, oldEnd, newText.slice(prefixLen, newEnd));
         textarea.selectionStart = textarea.selectionEnd = result.cursor;
-        textarea.dispatchEvent(new Event('input'));
-        textarea.focus();
     }
 
     function applyTableOp(fn, arg) {

assets/editor/movie.js

@@ -49,22 +49,28 @@ window.EditorMovie = (function () {
         return out.join('\n');
     }
 
+    function replaceRange(ta, start, end, text) {
+        ta.focus();
+        ta.selectionStart = start;
+        ta.selectionEnd = end;
+        document.execCommand('insertText', false, text);
+    }
+
     function insertOrReplace(ta, markup) {
         var t = ta.value || '';
         var b = t.indexOf(BEGIN);
         var e = t.indexOf(END);
         if (b !== -1 && e !== -1 && e > b) {
-            ta.value = t.slice(0, b) + markup + t.slice(e + END.length);
+            replaceRange(ta, b, e + END.length, markup);
         } else {
             var h = t.match(/^#{1,6}\s+.+?\s*$/m);
             if (h) {
                 var idx = t.indexOf(h[0]) + h[0].length;
-                ta.value = t.slice(0, idx) + '\n\n' + markup + t.slice(idx);
+                replaceRange(ta, idx, idx, '\n\n' + markup);
             } else {
-                ta.value = markup + (t ? '\n\n' + t : '');
+                replaceRange(ta, 0, 0, t ? markup + '\n\n' : markup);
             }
         }
-        ta.dispatchEvent(new Event('input'));
     }
 
     function fetchMovie(title, year) {