Huh, I've never had this problem on KWin before.
I've only had #8870, which this still doesn't fix... Maybe KDE 6 will? :/

Can someone test on Windows please?

X11 (via XWayland) is unaffected, has the correct windows size both before and after.

Maybe this fixes #15157 though...?

I'm not sure about #15157, the window is the same size, but the image seems to have less gross filtering.

before	after

Is there a proper pixel art swf you can point me to?

Huh, I've never had this problem on KWin before.

I assume if you run it with WAYLAND_DEBUG=1 you'll see a xdg_toplevel@*configure when ruffle resizes itself? This doesn't happen on mutter, I assume it's because it is a request from the compositor to change size (drag resize) rather than a notification that it has resized.

Huh, weird.

Not quite pixel art, but https://z0r.de/L/z0r-de_420.swf for example is relatively small, and should be pixelated (possibly due to #10103, but whatever).
So it's easy to see if the UI is scaled incorrectly, and/or if the window content is stretched by the compositor.

Yes, I see those requests. So maybe it's KWin who insists on this "factory-default" size too much?

attila ~/projects/ruffle $ git checkout origin/master -f
HEAD is now at 9dfc6dc22 fix(deps): update rust dependencies
attila ~/projects/ruffle $ WAYLAND_DEBUG=1 cargo run -- 'https://z0r.de/L/z0r-de_420.swf' 2>&1 | grep -P 'xdg_toplevel@.*configure'
[2244621.129] xdg_toplevel@29.configure_bounds(1920, 1016)
[2244621.135] xdg_toplevel@29.configure(0, 0, array[0])
[2245301.342] xdg_toplevel@29.configure(800, 600, array[4])
[2245625.470] xdg_toplevel@29.configure(800, 600, array[4])
attila ~/projects/ruffle $ git checkout ColinKinloch/wl_resize_gui -f
Previous HEAD position was 9dfc6dc22 fix(deps): update rust dependencies
HEAD is now at 0cdde301a desktop: Fix gui scale on launch on wayland
attila ~/projects/ruffle $ WAYLAND_DEBUG=1 cargo run -- 'https://z0r.de/L/z0r-de_420.swf' 2>&1 | grep -P 'xdg_toplevel@.*configure'
[2287138.117] xdg_toplevel@29.configure_bounds(1920, 1016)
[2287138.120] xdg_toplevel@29.configure(0, 0, array[0])
[2287811.485] xdg_toplevel@29.configure(800, 600, array[4])
[2288157.708] xdg_toplevel@29.configure(800, 600, array[4])
attila ~/projects/ruffle $ 

This bug doesn't manifest if the swf is passed as an argument. I assume ruffle creates the windows/gui if it's already got the file.

On my machine, it looks and behaves exactly the same way before and after this PR, no matter if I supply the SWF on the command line, or in the "Open" dialog.
On your screenshots, the "after" really does look better, so I'm not against this change.
I'll only have time to look at the code later though.

Last one, I promise. 😄

before
after

I had a look at it in a nested kwin instance.
Pretty weird, it goes from the initial 800x600 to the requested size (324x216) on load then returns to 800x600 for some reason. 🤷

This is both on master and my branch.

Yep, that's what I'm seeing too. Sad.
Thank you for checking!
I've also checked the fresh Fedora 40 Beta with KDE Plasma 6 - had the exact same behavior... :(

More testing. The size reset issue seems to also exist in egui.

And here's this MR a nested weston session:

before	after

Current nightly on Arch Plasma 6 seems to keep the regular 1x GUI scale. Or is it not supposed to stay at 1x?

What's the test case that's supposed to trigger this?

What I said about Plasma 6 is not a matter of GUI scale, simply window size not matching the content size (as opposed to being reset to 800x600). So these are two related, but not exactly the same, issues.

Ah, right, testing in Weston, I get this before (currently):

And with this PR:

It's hard to see, but the content matches the window in size just a little bit better! 😄

EDIT: Oh, sorry, this is basically #15781 (comment) ... How did I not realize...?