When a window is resized AppKit groups all updates to the view frames and corresponding layer bounds, so that the result of the resize is visually atomic, but this only works for the main thread. http://openradar.appspot.com/radar?id=4990815088672768 When a separate thread renders to one of the views in the window, it may result in the view and its layer updating its bounds visually before the resize has been visually reflected for the window itself and its border. To ensure visually atomic updates, we disable all screen updates for the process during resizing. This is the same workaround used by e.g. the NSOpenPanel class, which renders the content of the view out of process, and by Chromium for a similar use-case: https://chromium-review.googlesource.com/c/chromium/src/+/798774 Ideally we'd do this only for the window that is being resized, but there's no known API to do that. The deprecated [NSWindow disableScreenUpdatesUntilFlush] is a no-op these days, and used NSDisableScreenUpdates internally anyways). Fixes: QTBUG-69321 Change-Id: I84de714782278f2e0b2b2e1eb245c30810cb3023 Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io> Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>