1 files changed, 25 insertions, 1 deletions
diff --git a/src/plugins/styles/mac/qmacstyle_mac.mm b/src/plugins/styles/mac/qmacstyle_mac.mm
index fd268e3787..f0bc51b796 100644
--- a/src/plugins/styles/mac/qmacstyle_mac.mm
+++ b/src/plugins/styles/mac/qmacstyle_mac.mm
@@ -236,6 +236,18 @@ static QLinearGradient titlebarGradientInactive()
 }
 
 #if QT_CONFIG(tabwidget)
+/*
+    Since macOS 10.14 AppKit is using transparency more extensively, especially for the
+    dark theme. Inactive buttons, for example, are semi-transparent. And we use them to
+    draw tab widget's tab bar. The combination of NSBox (also a part of tab widget)
+    and these transparent buttons gives us an undesired side-effect: an outline of
+    NSBox is visible through transparent buttons. To avoid this, we have this hack below:
+    we clip the area where the line would be visible through the buttons. The area we
+    want to clip away can be described as an intersection of the option's rect and
+    the tab widget's tab bar rect. But some adjustments are required, since those rects
+    are anyway adjusted during the rendering and they are not exactly what you'll see on
+    the screen. Thus this switch-statement inside.
+*/
 static void clipTabBarFrame(const QStyleOption *option, const QMacStyle *style, CGContextRef ctx)
 {
     Q_ASSERT(option);
@@ -246,7 +258,19 @@ static void clipTabBarFrame(const QStyleOption *option, const QMacStyle *style,
         QTabWidget *tabWidget = qobject_cast<QTabWidget *>(option->styleObject);
         Q_ASSERT(tabWidget);
 
-        const QRect tabBarRect = style->subElementRect(QStyle::SE_TabWidgetTabBar, option, tabWidget).adjusted(2, 2, -3, -2);
+        QRect tabBarRect = style->subElementRect(QStyle::SE_TabWidgetTabBar, option, tabWidget).adjusted(2, 0, -3, 0);
+        switch (tabWidget->tabPosition()) {
+        case QTabWidget::South:
+            tabBarRect.setY(tabBarRect.y() + tabBarRect.height() / 2);
+            break;
+        case QTabWidget::North:
+        case QTabWidget::West:
+            tabBarRect = tabBarRect.adjusted(0, 2, 0, -2);
+            break;
+        case QTabWidget::East:
+            tabBarRect = tabBarRect.adjusted(tabBarRect.width() / 2, 2, tabBarRect.width() / 2, -2);
+        }
+
         const QRegion clipPath = QRegion(option->rect) - tabBarRect;
         QVarLengthArray<CGRect, 3> cgRects;
         for (const QRect &qtRect : clipPath)