Issues with screen attach/detach/sleep/property changes

Events

NSScreen reconfiguration

• Is done in response to _NSCGSDisplayCGSNotificationHandler, which calls _NSApplicationReactToDisplayChanged.
  • This in turn calls [_NSScreenConfiguration invalidateConfigurationForReason:0x1], which refreshes NScreen.screens based on the new display info.
  • It then calls [NSApplication _reactToScreenInvalidation], which schedules a block to run on the next runloop pass.
    • This block calls [NSApplication _reactToScreenInvalidationImmediately:] when run, which among other things:
      • Updated depth for window, triggering NSWindowDidChangeBackingPropertiesNotification
      • Send screen change event to window, triggering NSWindowDidChangeScreenNotification
      • Moves windows if needed, triggering NSWindowDidMoveNotification
    • After all this, the block sends a NSApplicationDidChangeScreenParametersNotification

The problem with this flow from Qt's point of view is that the screen parameter notification comes in after the notifications for the window. We'd like to know about the screen update before all this happens, so we can process those events with an updated view of the screens.

CGDisplayReconfigurationCallBack

This is part of Quartz Display Services. It has two callbacks when a display change happens.

1. First the callback is called with the kCGDisplayBeginConfigurationFlag, via displayWillConfigNotifyProc
2. Then the callback is called with the changed flags after the change has happened, eg. kCGDisplayAddFlag, via displayConfigFinalizedProc

Both of these are delivered as events from the window server. The problem with this API is that the second callback is not guaranteed to happen after _NSCGSDisplayCGSNotificationHandler, at least not in practice, so we can't rely on the NSScreen array to be up to date at that point.

Debugging

Running this in a standalone terminal will show relevant debug output (assuming rasterwindow example used for testing):

sudo log stream --color always --level debug --process rasterwindow --style compact --type log --predicate 'subsystem == "com.apple.AppKit" || senderImagePath CONTAINS "QtCore"' | c++filt -n

Pass -NSScreenLoggingEnabled YES as arguments to the application to trigger screen configuration logging from AppKit.

Test cases

Switching GPUs

Changing resolution

Mirroring on/off

Display sleep

Changing main/primary display

Connecting/disconnecting external display