feat(createVirtualizer): add isVisible and proximity properties to VirtualItem, add filckering prevention check

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

@@ -14,6 +14,10 @@ export interface VirtualItem {
     end: number;
     /** Unique key for the item (for Svelte's {#each} keying) */
     key: string | number;
+    /** Whether the item is currently visible in the viewport */
+    isVisible: boolean;
+    /** Proximity of the item to the center of the viewport */
+    proximity: number;
 }
 
 /**
@@ -136,6 +140,8 @@ export function createVirtualizer<T>(
 
         let endIdx = startIdx;
         const viewportEnd = scrollOffset + containerHeight;
+        const viewportCenter = scrollOffset + (containerHeight / 2);
+
         while (endIdx < count && offsets[endIdx] < viewportEnd) {
             endIdx++;
         }
@@ -144,13 +150,31 @@ export function createVirtualizer<T>(
         const end = Math.min(count, endIdx + overscan);
 
         const result: VirtualItem[] = [];
+
         for (let i = start; i < end; i++) {
+            const itemStart = offsets[i];
+            const itemSize = measuredSizes[i] ?? options.estimateSize(i);
+            const itemEnd = itemStart + itemSize;
+
+            // Visibility check: Does the item overlap the viewport?
+            // const isVisible = itemStart < viewportEnd && itemEnd > scrollOffset;
+            // Fully visible
+            const isVisible = itemStart >= scrollOffset && itemEnd <= viewportEnd;
+
+            // Proximity calculation: 1.0 at center, 0.0 at edges
+            const itemCenter = itemStart + (itemSize / 2);
+            const distanceToCenter = Math.abs(viewportCenter - itemCenter);
+            const maxDistance = containerHeight / 2;
+            const proximity = Math.max(0, 1 - (distanceToCenter / maxDistance));
+
             result.push({
                 index: i,
-                start: offsets[i],
-                size: measuredSizes[i] ?? options.estimateSize(i),
-                end: offsets[i] + (measuredSizes[i] ?? options.estimateSize(i)),
+                start: itemStart,
+                size: itemSize,
+                end: itemEnd,
                 key: options.getItemKey?.(i) ?? i,
+                isVisible,
+                proximity,
             });
         }
 
@@ -207,23 +231,25 @@ export function createVirtualizer<T>(
             const index = parseInt(node.dataset.index || '', 10);
             const height = entry.borderBoxSize[0]?.blockSize ?? node.offsetHeight;
 
-            if (!isNaN(index) && measuredSizes[index] !== height) {
-                // 1. Stuff the measurement into a temporary buffer
+            if (!isNaN(index)) {
+                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
                     measurementBuffer[index] = height;
 
-                // 2. Schedule a single update for the next animation frame
+                    // Schedule a single update for the next animation frame
                     if (frameId === null) {
                         frameId = requestAnimationFrame(() => {
-                        // 3. Update the state once for all collected measurements
-                        // We use spread to trigger a single fine-grained update
                             measuredSizes = { ...measuredSizes, ...measurementBuffer };
-
-                        // 4. Reset the buffer
+                            // Reset the buffer
                             measurementBuffer = {};
                             frameId = null;
                         });
                     }
                 }
+            }
         });
 
         resizeObserver.observe(node);