Add PDF state persistence

src-tauri/assets/viewer/viewport-tracker.js

@@ -23,8 +23,21 @@ export class ViewportTracker {
   }
 
   _observe(root, pageWrappers) {
-    const notify = () => {
-      this._onVisibilityChange(new Set(this._bufferSet), new Set(this._visibleSet));
+    // Both observers update their respective sets and then schedule a single
+    // notification via rAF.  This prevents a stale notify when the two
+    // observers fire in separate microtasks for the same layout change —
+    // e.g. bufferObserver removes a page from bufferSet before visibleObserver
+    // has had a chance to also remove it from visibleSet, which would let
+    // reconcile() incorrectly tear down a still-visible canvas.
+    this._rafPending = null;
+    const scheduleNotify = () => {
+      if (this._rafPending !== null) return;
+      this._rafPending = requestAnimationFrame(() => {
+        this._rafPending = null;
+        // Hard invariant: every visible page must also be in the buffer.
+        for (const p of this._visibleSet) this._bufferSet.add(p);
+        this._onVisibilityChange(new Set(this._bufferSet), new Set(this._visibleSet));
+      });
     };
 
     this._visibleObserver = new IntersectionObserver(
@@ -34,7 +47,7 @@ export class ViewportTracker {
           if (e.isIntersecting) this._visibleSet.add(page);
           else this._visibleSet.delete(page);
         }
-        notify();
+        scheduleNotify();
       },
       { root, rootMargin: "0px", threshold: 0 }
     );
@@ -46,7 +59,7 @@ export class ViewportTracker {
           if (e.isIntersecting) this._bufferSet.add(page);
           else this._bufferSet.delete(page);
         }
-        notify();
+        scheduleNotify();
       },
       { root, rootMargin: "200% 0px", threshold: 0 }
     );