QQuickTableView: detect if sync view and sync child is out of sync

When syncView is requested to do a relayout, it will tell
the sync children to do relayout as well. A relayout alone
will not change the top-left delegate item in the viewport,
only layout the existing delegate items.

But as it turns out, it can happen that the relayout leaves
empty space in the viewport that needs to be refilled with
new rows and columns. This is typically the case if some of
the columns were resized smaller than what they used to be.
And this can change which cell ends up as top-left, or
move it to a different position.

We therefore need to extend the check in syncSyncView to also
include the _position_ of the top-left cell, and not only if
the top-left cell itself has changed.

As it stood, we also did a viewportOnly rebuild every time a
new row or column was flicked into the viewport. This is very
unnecessary, and slows down performance. Instead, we only need
to do this when if the sync child is told to do (at least)
a LayoutOnly. Only then can any of the visible rows or columns
have been resized.

Fixes: QTBUG-127809
Pick-to: 6.5 6.2
Change-Id: I815a47e9d6453871172ff7136f8cfbd51a5bc789
Reviewed-by: Santhosh Kumar <santhosh.kumar.selvaraj@qt.io>
(cherry picked from commit 25348bc0e67f019232aa70c5558988a0c17bd15e)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>
(cherry picked from commit a5d0cfe092cc8411469ef53df13f2dac12051bf4)

2 files changed, 105 insertions, 10 deletions

diff --git a/src/quick/items/qquicktableview.cpp b/src/quick/items/qquicktableview.cpp
index a11cf4ae14..ed0c0f6d39 100644
--- a/src/quick/items/qquicktableview.cpp
+++ b/src/quick/items/qquicktableview.cpp
@@ -4429,11 +4429,18 @@ void QQuickTableViewPrivate::syncSyncView()
         q->setRightMargin(syncView->rightMargin());
         updateContentWidth();
 
-        if (syncView->leftColumn() != q->leftColumn()) {
-            // The left column is no longer the same as the left
-            // column in syncView. This requires a rebuild.
-            scheduledRebuildOptions |= QQuickTableViewPrivate::RebuildOption::CalculateNewTopLeftColumn;
-            scheduledRebuildOptions.setFlag(RebuildOption::ViewportOnly);
+        if (scheduledRebuildOptions & RebuildOption::LayoutOnly) {
+            if (syncView->leftColumn() != q->leftColumn()
+                    || syncView->d_func()->loadedTableOuterRect.left() != loadedTableOuterRect.left()) {
+                // The left column is no longer the same, or at the same pos, as the left column in
+                // syncView. This can happen if syncView did a relayout that caused its left column
+                // to be resized so small that it ended up outside the viewport. It can also happen
+                // if the syncView loaded and unloaded columns after the relayout. We therefore need
+                // to sync our own left column and pos to be the same, which we do by rebuilding the
+                // whole viewport instead of just doing a plain LayoutOnly.
+                scheduledRebuildOptions |= QQuickTableViewPrivate::RebuildOption::CalculateNewTopLeftColumn;
+                scheduledRebuildOptions.setFlag(RebuildOption::ViewportOnly);
+            }
         }
     }
 
@@ -4444,11 +4451,18 @@ void QQuickTableViewPrivate::syncSyncView()
         q->setBottomMargin(syncView->bottomMargin());
         updateContentHeight();
 
-        if (syncView->topRow() != q->topRow()) {
-            // The top row is no longer the same as the top
-            // row in syncView. This requires a rebuild.
-            scheduledRebuildOptions |= QQuickTableViewPrivate::RebuildOption::CalculateNewTopLeftRow;
-            scheduledRebuildOptions.setFlag(RebuildOption::ViewportOnly);
+        if (scheduledRebuildOptions & RebuildOption::LayoutOnly) {
+            if (syncView->topRow() != q->topRow()
+                    || syncView->d_func()->loadedTableOuterRect.top() != loadedTableOuterRect.top()) {
+                // The top row is no longer the same, or at the same pos, as the top row in
+                // syncView. This can happen if syncView did a relayout that caused its top row
+                // to be resized so small that it ended up outside the viewport. It can also happen
+                // if the syncView loaded and unloaded rows after the relayout. We therefore need
+                // to sync our own top row and pos to be the same, which we do by rebuilding the
+                // whole viewport instead of just doing a plain LayoutOnly.
+                scheduledRebuildOptions |= QQuickTableViewPrivate::RebuildOption::CalculateNewTopLeftRow;
+                scheduledRebuildOptions.setFlag(RebuildOption::ViewportOnly);
+            }
         }
     }
 
diff --git a/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp b/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp
index bb425b5a6f..54d6b2dd16 100644
--- a/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp
+++ b/tests/auto/quick/qquicktableview/tst_qquicktableview.cpp
@@ -169,6 +169,8 @@ private slots:
     void checkSyncView_emptyModel();
     void checkSyncView_topLeftChanged();
     void checkSyncView_unloadHeader();
+    void checkSyncView_dontRelayoutWhileFlicking();
+    void checkSyncView_detectTopLeftPositionChanged();
     void delegateWithRequiredProperties();
     void checkThatFetchMoreIsCalledWhenScrolledToTheEndOfTable();
     void replaceModel();
@@ -3344,6 +3346,85 @@ void tst_QQuickTableView::checkSyncView_topLeftChanged()
     QCOMPARE(tableViewV->topRow(), tableView->topRow());
 }
 
+void tst_QQuickTableView::checkSyncView_dontRelayoutWhileFlicking()
+{
+    // Check that we don't do a full relayout in a sync child when
+    // a new row or column is flicked into the view. Normal load
+    // and unload of edges should suffice, equal to how the main
+    // TableView (syncView) does it.
+    LOAD_TABLEVIEW("syncviewsimple.qml");
+    GET_QML_TABLEVIEW(tableViewHV);
+
+    auto model = TestModelAsVariant(100, 100);
+    tableView->setModel(model);
+    tableViewHV->setModel(model);
+
+    tableView->setColumnWidthProvider(QJSValue());
+    tableView->setRowHeightProvider(QJSValue());
+    view->rootObject()->setProperty("delegateWidth", 50);
+    view->rootObject()->setProperty("delegateHeight", 50);
+
+    WAIT_UNTIL_POLISHED;
+
+    // To check that we don't do a relayout when flicking horizontally, we use a "trick"
+    // where we check the rebuildOptions when we receive the rightColumnChanged
+    // signal. If this signal is emitted as a part of a relayout, rebuildOptions
+    // would still be different from RebuildOption::None at that point.
+    bool columnFlickedIn = false;
+    connect(tableViewHV, &QQuickTableView::rightColumnChanged, [&] {
+        columnFlickedIn = true;
+        QCOMPARE(tableViewHVPrivate->rebuildOptions, QQuickTableViewPrivate::RebuildOption::None);
+    });
+
+    // We do the same for vertical flicking
+    bool rowFlickedIn = false;
+    connect(tableViewHV, &QQuickTableView::bottomRowChanged, [&] {
+        rowFlickedIn = true;
+        QCOMPARE(tableViewHVPrivate->rebuildOptions, QQuickTableViewPrivate::RebuildOption::None);
+    });
+
+    // Move the main tableview so that a new column is flicked in
+    tableView->setContentX(60);
+    QTRY_VERIFY(columnFlickedIn);
+
+    // Move the main tableview so that a new row is flicked in
+    tableView->setContentY(60);
+    QTRY_VERIFY(rowFlickedIn);
+}
+
+void tst_QQuickTableView::checkSyncView_detectTopLeftPositionChanged()
+{
+    // It can happen that, during a resize of columns or rows from using a float-based
+    // slider, that the position of the top-left delegate item is shifted a bit left or
+    // right because of rounding issues. And this again can over time, as you flick, make
+    // the loadedTableOuterRect get slightly out of sync in the sync child compared to the
+    // sync view. TableView will detect if this happens (in syncSyncView), and correct for
+    // it. And this test will test that it works.
+    LOAD_TABLEVIEW("syncviewsimple.qml");
+    GET_QML_TABLEVIEW(tableViewHV);
+
+    auto model = TestModelAsVariant(100, 100);
+    tableView->setModel(model);
+    tableViewHV->setModel(model);
+
+    WAIT_UNTIL_POLISHED;
+
+    // Writing an auto test to trigger this rounding issue is very hard. So to keep it
+    // simple, we cheat by just moving the loadedTableOuterRect directly, and
+    // check that the syncView child detects it, and corrects it, upon doing a
+    // forceLayout()
+    tableViewPrivate->loadedTableOuterRect.moveLeft(20);
+    tableViewPrivate->loadedTableOuterRect.moveTop(30);
+    tableViewPrivate->relayoutTableItems();
+    tableViewHV->forceLayout();
+
+    QCOMPARE(tableViewPrivate->loadedTableOuterRect.left(), 20);
+    QCOMPARE(tableViewHVPrivate->loadedTableOuterRect.left(), 20);
+
+    QCOMPARE(tableViewPrivate->loadedTableOuterRect.top(), 30);
+    QCOMPARE(tableViewHVPrivate->loadedTableOuterRect.top(), 30);
+}
+
 void tst_QQuickTableView::checkThatFetchMoreIsCalledWhenScrolledToTheEndOfTable()
 {
     LOAD_TABLEVIEW("plaintableview.qml");