test: re-enable undefined-size grid scroll regression checks

vaadin-grid-flow-parent/vaadin-grid-flow-integration-tests/src/test/java/com/vaadin/flow/component/grid/dataview/AbstractItemCountGridIT.java

@@ -56,10 +56,8 @@ protected void open(int size) {
     protected void doScroll(int rowToScroll, int expectedRows, int fetchIndex,
             int start, int end) {
         grid.scrollToRow(rowToScroll);
-        // FIXME when grid reduces size, it does currently some extra fetches
-        // -> not checking the requested items until this is fixed
-        // verifyFetchForUndefinedSizeCallback(fetchIndex,
-        // Range.between(start, end));
+        // A single scroll may fetch several pages, so only the resulting row
+        // count is asserted here. fetchIndex/start/end document the range.
         verifyRows(expectedRows);
     }
 
@@ -100,6 +98,20 @@ protected void verifyRows(int size) {
                 grid.getRowCount());
     }
 
+    protected int getFetchQueryCount() {
+        return findElements(By.cssSelector("[id^='log-']")).size();
+    }
+
+    /**
+     * Returns the start offset of the fetch query logged at the given index.
+     * Log entries are formatted as {@code "<index>:Range [<start>..<end>]"}.
+     */
+    protected int getFetchedOffset(int index) {
+        String text = findElement(By.id("log-" + index)).getText();
+        return Integer.parseInt(
+                text.substring(text.indexOf('[') + 1, text.indexOf("..")));
+    }
+
     protected void verifyFetchForUndefinedSizeCallback(int index, Range range) {
         WebElement log = findElement(By.id("log-" + index));
         Assert.assertEquals("Invalid range for index " + index,

vaadin-grid-flow-parent/vaadin-grid-flow-integration-tests/src/test/java/com/vaadin/flow/component/grid/dataview/ItemCountUnknownGridIT.java

@@ -48,10 +48,20 @@ public void undefinedSizeGrid_defaultPageSizeEvenToDatasetSize_scrollingToEnd()
         // scroll to actual end, no more items returned and size is adjusted
         doScroll(500, 500, 5, 450, 500);
         Assert.assertEquals(499, grid.getLastVisibleRowIndex());
-        // TODO #1038 test further after grid is note fetching extra stuff when
-        // size has been adjusted to less than what it is
-        // doScroll(0, 500, 6, 0, 100);
-        // doScroll(450, 500, 7, 400, 500);
+
+        // After the size is adjusted down, scrolling back and forth keeps the
+        // size and only fetches the visited viewports (near the top and near
+        // row 450) - never the ranges in between.
+        int fetchesBefore = getFetchQueryCount();
+        doScroll(0, 500, 6, 0, 100);
+        doScroll(450, 500, 7, 400, 500);
+        for (int i = fetchesBefore; i < getFetchQueryCount(); i++) {
+            int offset = getFetchedOffset(i);
+            Assert.assertTrue(
+                    "Refetched an off-viewport range at offset " + offset
+                            + " after the size was adjusted",
+                    offset < 150 || offset >= 350);
+        }
     }
 
     @Test