QMacStyle: workaround NSSliderCell's cached/stale geometry

It's a bit cheesy solution, but works as I've noticed first on
QSlider's with a ticks direction 'both'. Works because we were
already using numberOfTickMarks property to trigger a special
behavior for HIG non-compliant widgets. Works for our case too
- we trigger a geometry update.

Fixes: QTBUG-76811
Change-Id: I2cbf00d42d98e78519b281d138a2f74227ef5449
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>

src/plugins/styles/mac/qmacstyle_mac.mm

@@ -137,6 +137,8 @@
 #include <qpa/qplatformtheme.h>
 #include <QtGui/private/qcoregraphics_p.h>
 
+#include <cmath>
+
 QT_USE_NAMESPACE
 
 static QWindow *qt_getWindow(const QWidget *widget)
@@ -514,6 +516,37 @@ static bool setupSlider(NSSlider *slider, const QStyleOptionSlider *sl)
     return true;
 }
 
+static void fixStaleGeometry(NSSlider *slider)
+{
+    // If it's later fixed in AppKit, this function is not needed.
+    // On macOS Mojave we suddenly have NSSliderCell with a cached
+    // (and stale) geometry, thus its -drawKnob, -drawBarInside:flipped:,
+    // -drawTickMarks fail to render the slider properly. Setting the number
+    // of tickmarks triggers an update in geometry.
+
+    Q_ASSERT(slider);
+
+    if (QOperatingSystemVersion::current() < QOperatingSystemVersion::MacOSMojave)
+        return;
+
+    NSSliderCell *cell = slider.cell;
+    const NSRect barRect = [cell barRectFlipped:NO];
+    const NSSize sliderSize = slider.frame.size;
+    CGFloat difference = 0.;
+    if (slider.vertical)
+        difference = std::abs(sliderSize.height - barRect.size.height);
+    else
+        difference = std::abs(sliderSize.width - barRect.size.width);
+
+    if (difference > 6.) {
+        // Stale ...
+        const auto nOfTicks = slider.numberOfTickMarks;
+        // Non-zero, different from nOfTicks to force update
+        slider.numberOfTickMarks = nOfTicks + 10;
+        slider.numberOfTickMarks = nOfTicks;
+    }
+}
+
 static bool isInMacUnifiedToolbarArea(QWindow *window, int windowY)
 {
     QPlatformNativeInterface *nativeInterface = QGuiApplication::platformNativeInterface();
@@ -5318,6 +5351,8 @@ void QMacStyle::drawComplexControl(ComplexControl cc, const QStyleOptionComplex
 #endif
                 {
                     [slider calcSize];
+                    if (!hasDoubleTicks)
+                        fixStaleGeometry(slider);
                     NSSliderCell *cell = slider.cell;
 
                     const int numberOfTickMarks = slider.numberOfTickMarks;