QStyleSheet: never treat styled scrollbars as transient

If a style sheet is applied to a scrollbar, then we cannot treat it as
transient, as the QStyleSheetStyle doesn't implement any fade-in/out
animation logic. And we also need to set the overlap to 0 (in both
the style sheet and the macOS style) if the scrollbars are not
transient; otherwise the opaque scrollbar will be placed on top of the
content.

Since a style sheet might only apply to a scrollbar based on its
orientation, we also have to pass the style option through to all calls
of the styleHint function.

And since that function is also called from other QStyle implementations
in the macOS style, we have to make sure that we call styleHint() on the
widget's style to get the correct value based on the style sheet.

Fixes: QTBUG-63381
Change-Id: Ic67ce3a7cb5089f885dabfd5a1951e3029915446
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>
(cherry picked from commit f5105ea89a76d6051f058834d99385582cc61f85)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 8 insertions, 4 deletions

diff --git a/src/plugins/styles/mac/qmacstyle_mac.mm b/src/plugins/styles/mac/qmacstyle_mac.mm
index afa72b3f09..c95847851e 100644
--- a/src/plugins/styles/mac/qmacstyle_mac.mm
+++ b/src/plugins/styles/mac/qmacstyle_mac.mm
@@ -2553,10 +2553,13 @@ int QMacStyle::pixelMetric(PixelMetric metric, const QStyleOption *opt, const QW
     case PM_ToolBarFrameWidth:
         ret = 1;
         break;
-    case PM_ScrollView_ScrollBarOverlap:
-        ret = [NSScroller preferredScrollerStyle] == NSScrollerStyleOverlay ?
-               pixelMetric(PM_ScrollBarExtent, opt, widget) : 0;
+    case PM_ScrollView_ScrollBarOverlap: {
+        const QStyle *realStyle = widget ? widget->style() : proxy();
+        ret = realStyle->styleHint(SH_ScrollBar_Transient, opt, widget)
+            ? realStyle->pixelMetric(PM_ScrollBarExtent, opt, widget)
+            : 0;
         break;
+    }
     default:
         ret = QCommonStyle::pixelMetric(metric, opt, widget);
         break;
@@ -5192,7 +5195,8 @@ void QMacStyle::drawComplexControl(ComplexControl cc, const QStyleOptionComplex
             const auto cocoaSize = d->effectiveAquaSizeConstrain(opt, widget);
             const CGFloat maxExpandScale = expandedKnobWidths[cocoaSize] / knobWidths[cocoaSize];
 
-            const bool isTransient = proxy()->styleHint(SH_ScrollBar_Transient, opt, widget);
+            const QStyle *realStyle = widget ? widget->style() : proxy();
+            const bool isTransient = realStyle->styleHint(SH_ScrollBar_Transient, opt, widget);
             if (!isTransient)
                 d->stopAnimation(opt->styleObject);
             bool wasActive = false;