2 files changed, 79 insertions, 4 deletions

diff --git a/src/widgets/widgets/qtabbar.cpp b/src/widgets/widgets/qtabbar.cpp
index 74e94cac4a..837874aebf 100644
--- a/src/widgets/widgets/qtabbar.cpp
+++ b/src/widgets/widgets/qtabbar.cpp
@@ -534,7 +534,6 @@ void QTabBarPrivate::layoutTabs()
 
     if (useScrollButtons && tabList.size() && last > available) {
         const QRect scrollRect = normalizedScrollRect(0);
-        scrollOffset = -scrollRect.left();
 
         Q_Q(QTabBar);
         QStyleOption opt;
@@ -571,10 +570,9 @@ void QTabBarPrivate::layoutTabs()
         leftB->show();
 
         rightB->setGeometry(scrollButtonRightRect);
-        rightB->setEnabled(last - scrollOffset > scrollRect.x() + scrollRect.width());
+        rightB->setEnabled(last + scrollRect.left() > scrollRect.x() + scrollRect.width());
         rightB->show();
     } else {
-        scrollOffset = 0;
         rightB->hide();
         leftB->hide();
     }
@@ -591,6 +589,11 @@ QRect QTabBarPrivate::normalizedScrollRect(int index)
     // tab bar itself is in a different orientation.
 
     Q_Q(QTabBar);
+    // If scrollbuttons are not visible, then there's no tear either, and
+    // the entire widget is the scroll rect.
+    if (leftB->isHidden())
+        return verticalTabs(shape) ? q->rect().transposed() : q->rect();
+
     QStyleOptionTab opt;
     q->initStyleOption(&opt, currentIndex);
     opt.rect = q->rect();
@@ -670,16 +673,18 @@ int QTabBarPrivate::hoveredTabIndex() const
 void QTabBarPrivate::makeVisible(int index)
 {
     Q_Q(QTabBar);
-    if (!validIndex(index) || leftB->isHidden())
+    if (!validIndex(index))
         return;
 
     const QRect tabRect = tabList.at(index)->rect;
     const int oldScrollOffset = scrollOffset;
     const bool horiz = !verticalTabs(shape);
+    const int available = horiz ? q->width() : q->height();
     const int tabStart = horiz ? tabRect.left() : tabRect.top();
     const int tabEnd = horiz ? tabRect.right() : tabRect.bottom();
     const int lastTabEnd = horiz ? tabList.constLast()->rect.right() : tabList.constLast()->rect.bottom();
     const QRect scrollRect = normalizedScrollRect(index);
+    const QRect entireScrollRect = normalizedScrollRect(0); // ignore tears
     const int scrolledTabBarStart = qMax(1, scrollRect.left() + scrollOffset);
     const int scrolledTabBarEnd = qMin(lastTabEnd - 1, scrollRect.right() + scrollOffset);
 
@@ -689,6 +694,12 @@ void QTabBarPrivate::makeVisible(int index)
     } else if (tabEnd > scrolledTabBarEnd) {
         // Tab is outside on the right, so scroll right.
         scrollOffset = tabEnd - scrollRect.right();
+    } else if (scrollOffset + entireScrollRect.width() > lastTabEnd + 1) {
+        // there's space on the right
+        scrollOffset = lastTabEnd - entireScrollRect.width() + 1;
+    } else if (available >= lastTabEnd) {
+        // the entire tabbar fits, reset scroll
+        scrollOffset = 0;
     }
 
     leftB->setEnabled(scrollOffset > -scrollRect.left());
@@ -1778,6 +1789,8 @@ void QTabBar::resizeEvent(QResizeEvent *)
     Q_D(QTabBar);
     if (d->layoutDirty)
         updateGeometry();
+
+    // when resizing, we want to keep the scroll offset as much as possible
     d->layoutTabs();
 
     d->makeVisible(d->currentIndex);
diff --git a/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp b/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp
index 1d7e999b10..8fd7b5c4b2 100644
--- a/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp
+++ b/tests/auto/widgets/widgets/qtabbar/tst_qtabbar.cpp
@@ -94,6 +94,9 @@ private slots:
     void hoverTab_data();
     void hoverTab();
 
+    void resizeKeepsScroll_data();
+    void resizeKeepsScroll();
+
 private:
     void checkPositions(const TabBar &tabbar, const QList<int> &positions);
 };
@@ -1346,5 +1349,64 @@ void tst_QTabBar::hoverTab()
     QCOMPARE(tabbar.styleOptions[1].state & QStyle::State_MouseOver, QStyle::State_None);
 }
 
+
+void tst_QTabBar::resizeKeepsScroll_data()
+{
+    QTest::addColumn<QTabBar::Shape>("tabShape");
+
+    QTest::addRow("North") << QTabBar::RoundedNorth;
+    QTest::addRow("East") << QTabBar::RoundedEast;
+    QTest::addRow("South") << QTabBar::RoundedSouth;
+    QTest::addRow("West") << QTabBar::RoundedWest;
+}
+
+void tst_QTabBar::resizeKeepsScroll()
+{
+    QFETCH(QTabBar::Shape, tabShape);
+
+    QTabBar tabBar;
+    TabBarScrollingProxyStyle proxyStyle;
+    tabBar.setStyle(&proxyStyle);
+
+    for (int i = 0; i < 10; ++i)
+        tabBar.addTab(u"Tab Number %1"_s.arg(i));
+
+    tabBar.setShape(tabShape);
+    tabBar.setUsesScrollButtons(true);
+
+    // resize to half
+    const QSize fullSize = tabBar.sizeHint();
+    const bool horizontal = fullSize.width() > fullSize.height();
+    if (horizontal)
+        tabBar.resize(fullSize.width() / 2, fullSize.height());
+    else
+        tabBar.resize(fullSize.width(), fullSize.height() / 2);
+
+    tabBar.show();
+    QVERIFY(QTest::qWaitForWindowExposed(&tabBar));
+
+    // select a tab outside, this will scroll
+    tabBar.setCurrentIndex(6);
+    // the first tab is now scrolled out
+    const int scrollOffset = horizontal
+                           ? tabBar.tabRect(0).left()
+                           : tabBar.tabRect(0).top();
+    QCOMPARE_LT(scrollOffset, 0);
+    // the current index is now fully visible, with margin on both sides
+    tabBar.setCurrentIndex(5);
+
+    // make the tab bar a bit larger, by the width of a tab
+    if (horizontal)
+        tabBar.resize(tabBar.width() + tabBar.tabRect(5).width(), tabBar.height());
+    else
+        tabBar.resize(tabBar.width(), tabBar.height() + tabBar.tabRect(5).height());
+
+    // this should not change the scroll
+    QCOMPARE(scrollOffset, horizontal
+                         ? tabBar.tabRect(0).left()
+                         : tabBar.tabRect(0).top());
+
+}
+
 QTEST_MAIN(tst_QTabBar)
 #include "tst_qtabbar.moc"