macOS: Avoid memory leak when using NSSlider for style drawing

To fix the broken sliders reported in QTBUG-98093 a workaround was
added by 4bee9cdc0ac4bbee7f061e8f6050d704032f6d0f where we would
call initWithFrame on an already initialized NSSlider.

This breaks the contract of object initialization in Objective-C,
as the class is free to allocate and prepare resources for the
instance without freeing previously acquired resources first.
As noted by the Object Initialization chapter of the Concepts
in Objective-C Programming guide, "Once an object is initialized,
you should not initialize it again.":

    https://tinyurl.com/objc-object-init

And as observed in QTBUG-112899, the additional initialization
resulted in a memory leak.

The other part of 4bee9cdc0ac4bbee7f061e8f6050d704032f6d0f
was that we now called startTrackingAt twice when drawing.
Both from setupSlider, for all consumers, and from
drawComplexControl, and as it turns out, this is the key
thing that "fixes" the pressed knob drawing of NSSlider.

For some reason, NSSlider needs the duplicate startTrackingAt
call both to draw the knob as pressed, and to not let one
drawing pass affect another drawing pass. This would benefit
from further investigation, but for now the removed leak
is an improvement.

Fixes: QTBUG-112899
Task-number: QTBUG-98093
Pick-to: 6.5
Change-Id: Ia7e6ef963910f1858d2fdb10e0323fc5bb3b2eda
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

1 files changed, 15 insertions, 15 deletions

diff --git a/src/plugins/styles/mac/qmacstyle_mac.mm b/src/plugins/styles/mac/qmacstyle_mac.mm
index 15dcea13bd..2bab19d682 100644
--- a/src/plugins/styles/mac/qmacstyle_mac.mm
+++ b/src/plugins/styles/mac/qmacstyle_mac.mm
@@ -429,12 +429,7 @@ static bool setupSlider(NSSlider *slider, const QStyleOptionSlider *sl)
     if (sl->minimum >= sl->maximum)
         return false;
 
-    // NSSlider seems to cache values based on tracking and the last layout of the
-    // NSView, resulting in incorrect knob rects that break the interaction with
-    // multiple sliders. So completely reinitialize the slider.
-    const auto controlSize = slider.controlSize;
-    [slider initWithFrame:sl->rect.toCGRect()];
-    slider.controlSize = controlSize;
+    slider.frame = sl->rect.toCGRect();
 
     slider.minValue = sl->minimum;
     slider.maxValue = sl->maximum;
@@ -465,14 +460,6 @@ static bool setupSlider(NSSlider *slider, const QStyleOptionSlider *sl)
     // the cell for its metrics and to draw itself.
     [slider layoutSubtreeIfNeeded];
 
-    if (sl->state & QStyle::State_Sunken) {
-        const CGRect knobRect = [slider.cell knobRectFlipped:slider.isFlipped];
-        CGPoint pressPoint;
-        pressPoint.x = CGRectGetMidX(knobRect);
-        pressPoint.y = CGRectGetMidY(knobRect);
-        [slider.cell startTrackingAt:pressPoint inView:slider];
-    }
-
     return true;
 }
 
@@ -5386,6 +5373,15 @@ void QMacStyle::drawComplexControl(ComplexControl cc, const QStyleOptionComplex
                 const CGRect knobRect = [slider.cell knobRectFlipped:slider.isFlipped];
                 pressPoint.x = CGRectGetMidX(knobRect);
                 pressPoint.y = CGRectGetMidY(knobRect);
+
+                // The only way to tell a NSSlider/NSSliderCell to render as pressed
+                // is to start tracking. But this API has some weird behaviors that
+                // we have to account for. First of all, the pressed state will not
+                // be visually reflected unless we start tracking twice. And secondly
+                // if we don't track twice, the state of one render-pass will affect
+                // the render pass of other sliders, even if we set up the shared
+                // NSSlider with a new slider value.
+                [slider.cell startTrackingAt:pressPoint inView:slider];
                 [slider.cell startTrackingAt:pressPoint inView:slider];
             }
 
@@ -5490,8 +5486,12 @@ void QMacStyle::drawComplexControl(ComplexControl cc, const QStyleOptionComplex
                 }
             });
 
-            if (isPressed)
+            if (isPressed) {
+                // We stop twice to be on the safe side, even if one seems to be enough.
+                // See startTracking above for why we do this.
+                [slider.cell stopTracking:pressPoint at:pressPoint inView:slider mouseIsUp:NO];
                 [slider.cell stopTracking:pressPoint at:pressPoint inView:slider mouseIsUp:NO];
+            }
         }
         break;
 #if QT_CONFIG(spinbox)