QTabBar: properly calc tab positions when changing tab position

The previous attempt to fix this bug did not work out well since it
tried too hard to avoid double calculations. Therefore restore the old
behavior and fix makeVisible() instead.
This amends 1082038bd89d0bbba4df0d9b3f8af3cd0a2f96f2

Pick-to: 6.6 6.5
Fixes: QTBUG-119366
Change-Id: I738a3cd3537ecf0983480d2f961f45f81d101bd9
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>

2 files changed, 64 insertions, 6 deletions

diff --git a/src/widgets/widgets/qtabbar.cpp b/src/widgets/widgets/qtabbar.cpp
index 982bdad7a8..7ddd611427 100644
--- a/src/widgets/widgets/qtabbar.cpp
+++ b/src/widgets/widgets/qtabbar.cpp
@@ -682,7 +682,10 @@ void QTabBarPrivate::makeVisible(int index)
     const int scrolledTabBarStart = qMax(1, scrollRect.left() + scrollOffset);
     const int scrolledTabBarEnd = qMin(lastTabEnd - 1, scrollRect.right() + scrollOffset);
 
-    if (tabStart < scrolledTabBarStart) {
+     if (available >= lastTabEnd) {
+        // the entire tabbar fits, reset scroll
+        scrollOffset = 0;
+    } else if (tabStart < scrolledTabBarStart) {
         // Tab is outside on the left, so scroll left.
         scrollOffset = tabStart - scrollRect.left();
     } else if (tabEnd > scrolledTabBarEnd) {
@@ -691,9 +694,6 @@ void QTabBarPrivate::makeVisible(int index)
     } else if (scrollOffset + entireScrollRect.width() > lastTabEnd + 1) {
         // fill any free space on the right without overshooting
         scrollOffset = qMax(0, lastTabEnd - entireScrollRect.width() + 1);
-    } else if (available >= lastTabEnd) {
-        // the entire tabbar fits, reset scroll
-        scrollOffset = 0;
     }
 
     leftB->setEnabled(scrollOffset > -scrollRect.left());
@@ -833,9 +833,14 @@ void QTabBarPrivate::refresh()
             pressedIndex = -1;
     }
 
-    layoutDirty = true;
-    if (q->isVisible())
+    if (!q->isVisible()) {
+        layoutDirty = true;
+    } else {
+        layoutTabs();
+        makeVisible(currentIndex);
         q->update();
+        q->updateGeometry();
+    }
 }
 
 /*!
diff --git a/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp b/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp
index 2e6ae0c4cd..08bfdb442e 100644
--- a/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp
+++ b/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp
@@ -102,6 +102,7 @@ private slots:
     void changeTabTextKeepsScroll();
     void settingCurrentTabBeforeShowDoesntScroll();
     void checkPositionsAfterShapeChange();
+    void checkScrollOffsetAfterTabRemoval();
 
 private:
     void checkPositions(const TabBar &tabbar, const QList<int> &positions);
@@ -1543,5 +1544,57 @@ void tst_QTabBar::checkPositionsAfterShapeChange()
     QVERIFY(opt.rect.top() > 0);
 }
 
+void tst_QTabBar::checkScrollOffsetAfterTabRemoval()
+{
+    QTabWidget tabWidget;
+    QTabBar *tabBar = tabWidget.tabBar();
+    for (int i = 0; i < 10; ++i)
+        tabWidget.addTab(new QWidget, u"Tab %1"_s.arg(i));
+    tabWidget.setTabPosition(QTabWidget::North);
+    tabWidget.resize(300, 300);
+    tabWidget.setCurrentIndex(0);
+    tabWidget.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&tabWidget));
+
+    auto *rightButton = tabBar->findChild<QAbstractButton *>(u"ScrollRightButton"_s);
+    auto *leftButton = tabBar->findChild<QAbstractButton *>(u"ScrollLeftButton"_s);
+    QVERIFY(leftButton);
+    QVERIFY(rightButton);
+    QVERIFY(rightButton->isEnabled());
+    QVERIFY(!leftButton->isEnabled());
+    // scroll to the right
+    tabBar->setCurrentIndex(9);
+    QVERIFY(!rightButton->isEnabled());
+    QVERIFY(leftButton->isEnabled());
+    // scroll to the center
+    tabBar->setCurrentIndex(2);
+    QVERIFY(rightButton->isEnabled());
+    QVERIFY(leftButton->isEnabled());
+
+    const auto getScrollOffset = [&]() -> int {
+        return static_cast<QTabBarPrivate *>(QObjectPrivate::get(tabBar))->scrollOffset;
+    };
+    // the scroll offset should not change when a tab right outside
+    // the scroll rect is removed
+    auto oldOffset = getScrollOffset();
+    tabWidget.removeTab(9);
+    QCOMPARE(getScrollOffset(), oldOffset);
+    // the scroll offset must change when a tab left outside
+    // the scroll rect is removed
+    oldOffset = getScrollOffset();
+    tabWidget.removeTab(0);
+    QVERIFY(getScrollOffset() < oldOffset);
+
+    // the scroll offset must change when there is empty
+    // place in the right after tab removal
+    oldOffset = getScrollOffset();
+    QVERIFY(oldOffset > 0);
+    for (int i : { 7, 6, 5, 4, 3 })
+        tabWidget.removeTab(i);
+    QCOMPARE(getScrollOffset(), 0);
+    QVERIFY(!rightButton->isVisible());
+    QVERIFY(!leftButton->isVisible());
+}
+
 QTEST_MAIN(tst_QTabBar)
 #include "tst_qtabbar.moc"