feat(createVirtualizer): slidthly improve batching with version trigger

src/shared/lib/helpers/createVirtualizer/createVirtualizer.svelte.ts

@@ -120,9 +120,11 @@ export function createVirtualizer<T>(
     // By wrapping the getter in $derived, we track everything inside it
     const options = $derived(optionsGetter());
 
-    // This derivation now tracks: count, measuredSizes, AND the data array itself
+    // This derivation now tracks: count, _version (for measuredSizes updates), AND the data array itself
     const offsets = $derived.by(() => {
         const count = options.count;
+        // Implicit dependency on version signal
+        const v = _version;
         const result = new Float64Array(count);
         let accumulated = 0;
         for (let i = 0; i < count; i++) {
@@ -144,6 +146,8 @@ export function createVirtualizer<T>(
         // We MUST read options.data here so Svelte knows to re-run
         // this derivation when the items array is replaced!
         const { count, data } = options;
+        // Implicit dependency
+        const v = _version;
         if (count === 0 || containerHeight === 0 || !data) return [];
 
         const overscan = options.overscan ?? 5;
@@ -318,6 +322,9 @@ export function createVirtualizer<T>(
 
     let measurementBuffer: Record<number, number> = {};
     let frameId: number | null = null;
+    // Signal to trigger updates when mutating measuredSizes in place
+    let _version = $state(0);
+
     /**
      * Svelte action to measure individual item elements for dynamic height support.
      *
@@ -334,18 +341,25 @@ export function createVirtualizer<T>(
             const height = entry.borderBoxSize[0]?.blockSize ?? node.offsetHeight;
 
             if (!isNaN(index)) {
+                // Accessing the version ensures we have the latest state if needed,
+                // though here we just read the raw object.
                 const oldHeight = measuredSizes[index];
+
                 // Only update if the height difference is significant (> 0.5px)
-                // This prevents "jitter" from focus rings or sub-pixel border changes
                 if (oldHeight === undefined || Math.abs(oldHeight - height) > 0.5) {
-                    // Stuff the measurement into a temporary buffer
+                    // Stuff the measurement into a temporary buffer to batch updates
                     measurementBuffer[index] = height;
 
                     // Schedule a single update for the next animation frame
                     if (frameId === null) {
                         frameId = requestAnimationFrame(() => {
-                            measuredSizes = { ...measuredSizes, ...measurementBuffer };
-                            // Reset the buffer
+                            // Mutation in place for performance
+                            Object.assign(measuredSizes, measurementBuffer);
+
+                            // Trigger reactivity
+                            _version += 1;
+
+                            // Reset buffer
                             measurementBuffer = {};
                             frameId = null;
                         });