From 2b52fe0e5f80d93574dba91cae79942b55aec114 Mon Sep 17 00:00:00 2001
From: ushkal <44223690+ushkal@users.noreply.github.com>
Date: Thu, 6 Jun 2019 14:27:15 +0300
Subject: [PATCH] fix(react-grid): change processing of last page in lazy load
 mode (#2067)

---
 .../virtual-table-state/helpers.test.ts       |  8 +++++++
 .../plugins/virtual-table-state/helpers.ts    | 21 +++++++++++--------
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.test.ts b/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.test.ts
index fc475f6eff..71dbdeb440 100644
--- a/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.test.ts
+++ b/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.test.ts
@@ -203,6 +203,14 @@ describe('VirtualTableState helpers', () => {
         expect(calculateRequestedRange(loadedInterval, newInterval, 399, pageSize))
           .toEqual({ start: 300, end: 500 });
       });
+
+      it('should correctly process the last page', () => {
+        const loadedInterval = createInterval(0, 200);
+        const newInterval = createInterval(1000, 1200);
+
+        expect(calculateRequestedRange(loadedInterval, newInterval, 1180, pageSize))
+          .toEqual({ start: 1000, end: 1200 });
+      });
     });
 
     describe('fast scroll', () => {
diff --git a/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.ts b/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.ts
index 7aa97e8cf6..62164d2374 100644
--- a/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.ts
+++ b/packages/dx-grid-core/src/plugins/virtual-table-state/helpers.ts
@@ -45,16 +45,19 @@ export const mergeRows: MergeRowsFn = (
 export const calculateRequestedRange: CalculateRequestedRangeFn = (
   loadedInterval, newRange, referenceIndex, pageSize,
 ) => {
-  if (Math.abs(loadedInterval.start - newRange.start) >= 2 * pageSize) {
-    const useFirstHalf = referenceIndex % pageSize < pageSize / 2;
-    const start = useFirstHalf
-      ? newRange.start
-      : newRange.start + pageSize;
-    const end = Math.min(newRange.end, start + 2 * pageSize);
-
-    return { start, end };
+  const isAdjacentPage = Math.abs(loadedInterval.start - newRange.start) < 2 * pageSize;
+  if (isAdjacentPage) {
+    return intervalUtil.difference(newRange, loadedInterval);
   }
-  return intervalUtil.difference(newRange, loadedInterval);
+
+  const useFirstHalf = referenceIndex % pageSize < pageSize / 2;
+  const isLastPage = intervalUtil.getLength(newRange) / pageSize < 3;
+  const start = useFirstHalf || isLastPage
+    ? newRange.start
+    : newRange.start + pageSize;
+  const end = Math.min(newRange.end, start + 2 * pageSize);
+
+  return { start, end };
 };
 
 export const rowToPageIndex: PureComputed<[number, number]> = (