bug#46827: Broken initial size of GTK3 frame

[martin rudalics]

> No frame resizes, the frame always stays maximized and undecorated,
> and the tab bar is not resized.

Let's check whether the frame code is affected.  With you original
scenario and also with the two scenarios where you successfully suppress
the flashes do

1: Eval (setq frame-size-history '(100))

2: Run the Gnus setup

3: Eval (progn (frame--size-history) (pop-to-buffer "*frame-size-history*"))

all in one and the same frame.  Then post the contents of the buffer
*frame-size-history* for each of them.

Thanks, martin