Tested again, Mixxx crashed with
....
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x400000005)
    frame #0: 0x00000001014ea4b4 QtGui`QImage::rect() const + 20
QtGui`QImage::rect:
->  0x1014ea4b4 <+20>: movl   0x4(%rax), %ecx
    0x1014ea4b7 <+23>: addl   %edx, %ecx
    0x1014ea4b9 <+25>: movl   0x8(%rax), %edx
    0x1014ea4bc <+28>: decl   %edx
Target 0: (mixxx) stopped.
(lldb) thread backtrace all
* thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=1, address=0x400000005)
  * frame #0: 0x00000001014ea4b4 QtGui`QImage::rect() const + 20
    frame #1: 0x00000001005ef90c mixxx`WSpinny::paintEvent(this=0x000000012a09ce30, e=<unavailable>) at wspinny.cpp:340 [opt]
    frame #2: 0x00000001014b835d QtGui`QWidget::event(QEvent*) + 2349
    frame #3: 0x00000001020cb042 QtOpenGL`QGLWidget::event(QEvent*) + 82
    frame #4: 0x0000000101460b0d QtGui`QApplicationPrivate::notify_helper(QObject*, QEvent*) + 189
    frame #5: 0x0000000101464d68 QtGui`QApplication::notify(QObject*, QEvent*) + 232
    frame #6: 0x000000010119f54c QtCore`QCoreApplication::notifyInternal(QObject*, QEvent*) + 124
    frame #7: 0x00000001014b5d9e QtGui`QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, int, QPainter*, QWidgetBackingStore*) + 1374
    frame #8: 0x000000010140535a QtGui`-[QCocoaView drawRect:] + 1786
    frame #9: 0x00007fff45cf1c21 AppKit`_NSViewDrawRect + 83
    frame #10: 0x00007fff4558ab38 AppKit`-[NSView _drawRect:clip:] + 1819
    frame #11: 0x00007fff455d2f62 AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 1735
    frame #12: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #13: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #14: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #15: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #16: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #17: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #18: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #19: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #20: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #21: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #22: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #23: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #24: 0x00007fff455d342f AppKit`-[NSView _recursiveDisplayAllDirtyWithLockFocus:visRect:] + 2964
    frame #25: 0x00007fff45588a52 AppKit`-[NSView _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] + 938
    frame #26: 0x00007fff455881db AppKit`-[NSThemeFrame _recursiveDisplayRectIfNeededIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:topView:] + 327
    frame #27: 0x00007fff45cf31d8 AppKit`-[NSView _oldDisplayRectIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:] + 2051
    frame #28: 0x00007fff45587261 AppKit`-[NSView _displayRectIgnoringOpacity:isVisibleRect:rectIsVisibleRectForView:] + 253
    frame #29: 0x00007fff455834b8 AppKit`-[NSView displayIfNeeded] + 1581
    frame #30: 0x00007fff45582e56 AppKit`-[NSWindow displayIfNeeded] + 321
    frame #31: 0x00007fff45d232d9 AppKit`___NSWindowGetDisplayCycleObserver_block_invoke.5902 + 308
    frame #32: 0x00007fff45582962 AppKit`__37+[NSDisplayCycle currentDisplayCycle]_block_invoke + 695
    frame #33: 0x00007fff52fb978f QuartzCore`CA::Transaction::run_commit_handlers(CATransactionPhase) + 49
    frame #34: 0x00007fff52fb8301 QuartzCore`CA::Transaction::commit() + 171
    frame #35: 0x00007fff45d3a658 AppKit`__65+[CATransaction(NSCATransaction) NS_setFlushesWithDisplayRefresh]_block_invoke + 283
    frame #36: 0x00007fff47e89427 CoreFoundation`__CFRUNLOOP_IS_CALLING_OUT_TO_AN_OBSERVER_CALLBACK_FUNCTION__ + 23
    frame #37: 0x00007fff47e8934f CoreFoundation`__CFRunLoopDoObservers + 511
    frame #38: 0x00007fff47e6bef8 CoreFoundation`__CFRunLoopRun + 1240
    frame #39: 0x00007fff47e6b787 CoreFoundation`CFRunLoopRunSpecific + 487
    frame #40: 0x00007fff47178e26 HIToolbox`RunCurrentEventLoopInMode + 286
    frame #41: 0x00007fff47178b96 HIToolbox`ReceiveNextEventCommon + 613
    frame #42: 0x00007fff47178914 HIToolbox`_BlockUntilNextEventMatchingListInModeWithFilter + 64
    frame #43: 0x00007fff45443f5f AppKit`_DPSNextEvent + 2085
    frame #44: 0x00007fff45bd9b4c AppKit`-[NSApplication(NSEvent) _nextEventMatchingEventMask:untilDate:inMode:dequeue:] + 3044
    frame #45: 0x00007fff45438d6d AppKit`-[NSApplication run] + 764
    frame #46: 0x00000001014195e0 QtGui`QEventDispatcherMac::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 1824
    frame #47: 0x000000010119e464 QtCore`QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) + 68
    frame #48: 0x000000010119e814 QtCore`QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) + 324
    frame #49: 0x00000001011a108c QtCore`QCoreApplication::exec() + 188
    frame #50: 0x00000001003ac132 mixxx`main [inlined] (anonymous namespace)::runMixxx(app=0x0000000100aeb040, args=<unavailable>) at main.cpp:53 [opt]
    frame #51: 0x00000001003ac040 mixxx`main(argc=<unavailable>, argv=0x00007ffeefbffb68) at main.cpp:127 [opt]
    frame #52: 0x00007fff6f821115 libdyld.dylib`start + 1
    frame #53: 0x00007fff6f821115 libdyld.dylib`start + 1
  thread #2
...

After applying the <PathMask> patch , Mixxx will not crash on my machine. At least it will not trigger the bug.