Lock image size of gtkglsink inside GtkFlowBoxChild?

I have a widget tree like this:

where the area-webcam will be replaced with a gtkglsink. To try to lock up the image size, I set size request for stack-img-source.

But when running the app, after replacing area-webcam, the gtkglsink just expand that area and make the UI not as I expect. Could you tell me how to fix it?

And this is the UI after the sink expands:

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