fix(frontend): loadFromState race condition + plugin stale closure (Issues #7, #8)

frontend/src/crdt/YjsDocument.ts

@@ -122,25 +122,33 @@ export class YjsDocument {
     bgConfigs?: BackgroundConfig[];
   }): void {
     this.doc.transact(() => {
-      this.data.elements.clear();
-      for (const el of state.elements) {
-        this.data.elements.set(el.id, el);
+      // Issue #7: Only clear and replace a map if the incoming array is non-empty.
+      // Clearing maps when data is empty wipes CRDT data from other connected clients.
+      if (state.elements.length > 0) {
+        this.data.elements.clear();
+        for (const el of state.elements) {
+          this.data.elements.set(el.id, el);
+        }
       }
-      this.data.layers.clear();
-      for (const layer of state.layers) {
-        this.data.layers.set(layer.id, layer);
+      if (state.layers.length > 0) {
+        this.data.layers.clear();
+        for (const layer of state.layers) {
+          this.data.layers.set(layer.id, layer);
+        }
       }
-      this.data.blocks.clear();
-      for (const block of state.blocks) {
-        this.data.blocks.set(block.id, block);
+      if (state.blocks.length > 0) {
+        this.data.blocks.clear();
+        for (const block of state.blocks) {
+          this.data.blocks.set(block.id, block);
+        }
       }
-      if (state.groups) {
+      if (state.groups && state.groups.length > 0) {
         this.data.groups.clear();
         for (const group of state.groups) {
           this.data.groups.set(group.id, group);
         }
       }
-      if (state.bgConfigs) {
+      if (state.bgConfigs && state.bgConfigs.length > 0) {
         this.data.bgConfig.clear();
         for (const cfg of state.bgConfigs) {
           // Use 'default' as the key since there is typically one background config