ux: FIX cursor updates to prevent cursor from jumping around while working together #21

app.py

@@ -399,9 +399,10 @@ function connect(){
       }
     } else if (msg.type === "update" && msg.ver > ver && msg.clientId !== clientId) {
       const {selectionStart:s, selectionEnd:e} = ta;
+      const oldText = ta.value;
       ta.value = msg.text; ver = msg.ver; updateGutter();
-      ta.selectionStart = Math.min(s, ta.value.length);
-      ta.selectionEnd   = Math.min(e, ta.value.length);
+      ta.selectionStart = adjustCursor(oldText, msg.text, s);
+      ta.selectionEnd   = adjustCursor(oldText, msg.text, e);
     } else if (msg.type === "peers_changed") {
       const el = $("#peers");
       el.textContent = msg.count;
@@ -439,6 +440,25 @@ async function copyToClipboard() {
   }
 }
 
+// Adjust cursor position after a remote text update.
+// Finds the single changed region (common prefix + suffix),
+// then shifts the cursor accordingly:
+//   - change is after cursor  → no movement
+//   - change is before cursor → shift by length delta
+//   - cursor was inside the changed region → place at end of new content
+function adjustCursor(oldText, newText, pos) {
+  let start = 0;
+  const minLen = Math.min(oldText.length, newText.length);
+  while (start < minLen && oldText[start] === newText[start]) start++;
+  if (pos <= start) return pos; // change is entirely after cursor
+  let oldEnd = oldText.length, newEnd = newText.length;
+  while (oldEnd > start && newEnd > start && oldText[oldEnd - 1] === newText[newEnd - 1]) {
+    oldEnd--; newEnd--;
+  }
+  if (pos >= oldEnd) return pos + (newEnd - oldEnd); // change is before cursor
+  return newEnd; // cursor was inside changed region
+}
+
 connect();
 
 // Handle Tab key to insert 4 spaces instead of navigation