MAYA-131609: Tabs go missing when changing direction

With commit 1082038 from Qt R&D, we were still able to reproduce the missing tabs issue when a tab was removed with tab bar scrolled to the right. While I understand that the problem arose because the geometry was not ready when the API was called, I believe the scroll offset calculation could be rewritten to handle various scenarios, regardless of how and when this API is used. My change reorders the existing logic and achieves the following:

Resets the scroll offset if there is enough space to fully display the tab bar before checking other conditions.
If there is not enough space, it clamps the adjusted scrollOffset value within the limits.
I have added a test case to demonstrate the issue we observed with the original commit.

src/widgets/widgets/qtabbar.cpp

@@ -680,20 +680,26 @@ 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) {
-        // Tab is outside on the left, so scroll left.
-        scrollOffset = tabStart - scrollRect.left();
-    } else if (tabEnd > scrolledTabBarEnd) {
-        // Tab is outside on the right, so scroll right.
-        scrollOffset = qMax(0, tabEnd - scrollRect.right());
-    } 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
+    if (available >= lastTabEnd) {
+        // the entire tabbar fits, reset scroll offset.
         scrollOffset = 0;
+    } else {
+        if (tabStart < scrolledTabBarStart) {
+            // Tab is partially occluded on the left, adjust offset to make tab fully visible.
+            scrollOffset = tabStart - scrollRect.left();
+        } else if (tabEnd > scrolledTabBarEnd) {
+            // Tab is partially occluded on the right, adjust offset to make tab fully visible.
+            scrollOffset = tabEnd - scrollRect.right();
+        }
+
+        if (scrollOffset + entireScrollRect.width() > lastTabEnd + 1) {
+            // fill any free space on the right without overshooting.
+            scrollOffset = lastTabEnd - entireScrollRect.width() + 1;
+        }
     }
 
+    scrollOffset = qMax(0, scrollOffset);
+
     leftB->setEnabled(scrollOffset > -scrollRect.left());
     rightB->setEnabled(scrollOffset < lastTabEnd - scrollRect.right());
 

tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp

@@ -100,6 +100,7 @@ private slots:
     void resizeKeepsScroll();
     void changeTabTextKeepsScroll();
     void settingCurrentTabBeforeShowDoesntScroll();
+    void checkScrollResetAfterTabRemoval();
 
 private:
     void checkPositions(const TabBar &tabbar, const QList<int> &positions);
@@ -1482,5 +1483,50 @@ void tst_QTabBar::settingCurrentTabBeforeShowDoesntScroll()
     QCOMPARE_GT(getScrollOffset(), 0);
 }
 
+void tst_QTabBar::checkScrollResetAfterTabRemoval()
+{
+    class TabWidget : public QTabWidget
+    {
+    public:
+        using QTabWidget::QTabWidget;
+        using QTabWidget::setTabBar;
+    };
+
+    TabWidget tabWidget;
+    QTabBar tabBar;
+    tabBar.setUsesScrollButtons(true);
+
+    tabWidget.setTabBar(&tabBar);
+    for (int i = 0; i < 6; ++i)
+        tabWidget.addTab(new QWidget, u"Tab %1"_s.arg(i));
+    tabWidget.setTabPosition(QTabWidget::North);
+    tabWidget.setCurrentIndex(0);
+    tabWidget.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&tabWidget));
+
+    QVERIFY(tabWidget.tabBar()->tabRect(0).intersects(tabWidget.tabBar()->rect()));
+    auto *rightButton = tabBar.findChild<QAbstractButton*>(u"ScrollRightButton"_s);
+
+    QVERIFY(rightButton);
+    QVERIFY(rightButton->isEnabled());
+    // simulate a right button click to increase the scroll offset.
+    for (int i = 0; i < 2; i++)
+        rightButton->click();
+
+    const auto getScrollOffset = [&]() -> int {
+        return static_cast<QTabBarPrivate *>(QObjectPrivate::get(&tabBar))->scrollOffset;
+    };
+
+    // scroll offset is expected to be greater than 0.
+    QCOMPARE_GT(getScrollOffset(), 0);
+
+    QVERIFY(!tabWidget.tabBar()->tabRect(0).intersects(tabWidget.tabBar()->rect()));
+    // try removing the last tab and check if the scroll offset is reset to 0, to show the first tab.
+    tabWidget.removeTab(5);
+
+    // scroll offset is expected to be reset to 0.
+    QCOMPARE(getScrollOffset(), 0);
+}
+
 QTEST_MAIN(tst_QTabBar)
 #include "tst_qtabbar.moc"