fix(virtual-core): expand range in masonry layouts to catch items from all lanes (#937)

Author: soren121

.changeset/gentle-cloths-dance.md

@@ -0,0 +1,5 @@
+---
+'@tanstack/virtual-core': patch
+---
+
+fix(virtual-core): expand range in masonry layouts to catch items from all lanes

packages/virtual-core/src/index.ts

@@ -1036,16 +1036,25 @@ export class Virtualizer<
 
     let end: number
     // If there are no measurements, set the end to paddingStart
+    // If there is only one lane, use the last measurement's end
+    // Otherwise find the maximum end value among all measurements
     if (measurements.length === 0) {
       end = this.options.paddingStart
+    } else if (this.options.lanes === 1) {
+      end = measurements[measurements.length - 1]?.end ?? 0
     } else {
-      // If lanes is 1, use the last measurement's end, otherwise find the maximum end value among all measurements
-      end =
-        this.options.lanes === 1
-          ? (measurements[measurements.length - 1]?.end ?? 0)
-          : Math.max(
-              ...measurements.slice(-this.options.lanes).map((m) => m.end),
-            )
+      const endByLane = Array<number | null>(this.options.lanes).fill(null)
+      let endIndex = measurements.length - 1
+      while (endIndex > 0 && endByLane.some((val) => val === null)) {
+        const item = measurements[endIndex]!
+        if (endByLane[item.lane] === null) {
+          endByLane[item.lane] = item.end
+        }
+
+        endIndex--
+      }
+
+      end = Math.max(...endByLane.filter((val): val is number => val !== null))
     }
 
     return Math.max(
@@ -1121,14 +1130,35 @@ function calculateRange({
   )
   let endIndex = startIndex
 
-  while (
-    endIndex < lastIndex &&
-    measurements[endIndex]!.end < scrollOffset + outerSize
-  ) {
-    endIndex++
-  }
+  if (lanes === 1) {
+    while (
+      endIndex < lastIndex &&
+      measurements[endIndex]!.end < scrollOffset + outerSize
+    ) {
+      endIndex++
+    }
+  } else if (lanes > 1) {
+    // Expand forward until we include the visible items from all lanes
+    // which are closer to the end of the virtualizer window
+    const endPerLane = Array(lanes).fill(0)
+    while (
+      endIndex < lastIndex &&
+      endPerLane.some((pos) => pos < scrollOffset + outerSize)
+    ) {
+      const item = measurements[endIndex]!
+      endPerLane[item.lane] = item.end
+      endIndex++
+    }
+
+    // Expand backward until we include all lanes' visible items
+    // closer to the top
+    const startPerLane = Array(lanes).fill(scrollOffset + outerSize)
+    while (startIndex > 0 && startPerLane.some((pos) => pos >= scrollOffset)) {
+      const item = measurements[startIndex]!
+      startPerLane[item.lane] = item.start
+      startIndex--
+    }
 
-  if (lanes > 1) {
     // Align startIndex to the beginning of its lane
     startIndex = Math.max(0, startIndex - (startIndex % lanes))
     // Align endIndex to the end of its lane