Embedded QWindow on X11: black background
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In Qt 6.8 I'm trying to embed a QWindow (specifically a QQuickView) inside a parent Qt window. The embedded window is properly transparent when it's a floating (non-embedded) window. On MacOS and Windows it has proper transparency when it's embedded, but on X11 it's "transparent", but instead of showing through to the parent window, it's drawn on top of a black rectangle (for example, if I set the background color to semi-transparent red, that just tints the black background).
macOS/Windows:
X11 (ignore round buttons, this is an older build):
I've tried manually setting the alpha buffer size to 8 as seen in some Qt docs, but that made no difference.... probably because window transparency is working, but it seems like there's another issue specific to embedding on X11.
Is there a fix for this?