Crash during window resize when using QQuickWidget

When using a QQuickWidget (at least on macOS, unknown Windows), during window resize can trigger a crash due to the composited window buffer being deallocated whilst uploading to the GPU is still in flight. The Metal validation layer outputs

-[MTLDebugDevice notifyExternalReferencesNonZeroOnDealloc:]:2885: failed assertion `The following Metal object is being destroyed while still required to be alive by the command buffer 0x7fb5328a5200 (label: <no label set>):
<MTLToolsObject: 0x60000140d500> -> <GFXAAMD_MtlBuffer: 0x7fb5314413c0>
    label = <none> 
    length = 20401152 
    cpuCacheMode = MTLCPUCacheModeDefaultCache 
    storageMode = MTLStorageModeShared 
    hazardTrackingMode = MTLHazardTrackingModeTracked 
    resourceOptions = MTLResourceCPUCacheModeDefaultCache MTLResourceStorageModeShared MTLResourceHazardTrackingModeTracked  
    purgeableState = MTLPurgeableStateNonVolatile' 

This is more easily reproduced with heavier interfaces (we can reproduce with a ~1700 line UI file) by rapidly resizing for a few seconds. Reducing the complexity makes it more difficult but not impossible to reproduce, suggesting a timing issue where the runtime is not waiting the required amount of time (for example, by not correctly calling [MTLCommandBuffer waitUntilCompleted], or not synchronizing MTLSharedEvent before releasing the resource) for an upload to complete before freeing the resource.