My understanding is that promoting a window to hardware overlay is only supported on Kaby Lake and later Intel integrated graphics, and there it's a heuristic,

My understanding is that promoting a window to hardware overlay is only supported on Kaby Lake and later Intel integrated graphics, and there it's a heuristic, so there's no way to guarantee getting it. You do have to be in flip mode, but in flip mode smooth resizing can't be done without artifacts. Currently druid downgrades to direct2d hwnd render targets during a live resize, but this feels hacky and is likely creating other problems. I've spent a fair amount of time investigating this and have a mostly written blog post on it, but right now I'm kind of sick of the topic - it's a good illustration of how easily software evolves into stuff that's complex and broken.