fix: use server content echo for remote edits instead of delta application

templates/document_edit.html

···
     }
 
     function applyRemoteEdit(msg) {
-        // msg may be a full-content string (legacy sync path) or an object with deltas.
+        // msg may be a full-content string (legacy sync path) or an object with content.
         if (applyingRemote) return;
         applyingRemote = true;
         try {
-            let content = typeof msg === 'string' ? msg : msg.content;
-            const deltas = (typeof msg === 'object' && msg.deltas) ? msg.deltas : null;
+            // Always use the server's authoritative content echo for applying remote edits.
+            // Attempting to apply deltas directly is unreliable because the positions in
+            // the delta batch are relative to the sender's pre-change document, not the
+            // receiver's current state (which may have diverged).
+            const content = typeof msg === 'string' ? msg : msg.content;
+            if (!content) return;
 
             if (currentMode === 'source' && cmView) {
-                if (deltas && deltas.length > 0) {
-                    // Apply granular deltas as a single CodeMirror dispatch.
-                    // All from/to positions are relative to the original document
-                    // (before any delta in this batch), which is what CodeMirror
-                    // expects when changes are batched in a single dispatch call.
-                    const docLen = cmView.state.doc.length;
-                    const changes = deltas.map(d => ({
-                        from: Math.min(d.from < 0 ? 0 : d.from, docLen),
-                        to:   Math.min(d.to   < 0 ? docLen : d.to, docLen),
-                        insert: d.insert || '',
-                    }));
-                    cmView.dispatch({
-                        changes,
-                        annotations: [remoteEditAnnotation.of(true)],
-                    });
-                    // Use the server's full-content echo as a divergence check.
-                    if (content && cmView.state.doc.toString() !== content) {
-                        // State diverged — fall back to full replacement.
-                        cmView.dispatch({
-                            changes: { from: 0, to: cmView.state.doc.length, insert: content },
-                            annotations: [remoteEditAnnotation.of(true)],
-                        });
-                    }
-                } else if (content && cmView.state.doc.toString() !== content) {
-                    // Legacy full-replacement path (sync message or old server).
+                if (cmView.state.doc.toString() !== content) {
                     cmView.dispatch({
                         changes: { from: 0, to: cmView.state.doc.length, insert: content },
                         annotations: [remoteEditAnnotation.of(true)],
                     });
                 }
-                if (content) updatePreview(content);
-            } else if (currentMode === 'rich' && milkdownEditor && content) {
+                updatePreview(content);
+            } else if (currentMode === 'rich' && milkdownEditor) {
                 createMilkdownEditor(content);
             }
         } finally {