Wrong window size reported to Wayland clients

This is an issue seen in a downstream project, and all debugging and testing seems to indicate it's a Qt problem. See https://github.com/keepassxreboot/keepassxc/issues/8293

When resizing a window to a smaller size, multiple elements are cropped rather than shrinking to fit the window. After inspecting with `gammaray`, it seems that Qt reports the wrong window size, so the client is just rendering assuming a larger surface (hence why content gets cropped).

In this particular case, the window width is reported correctly down to the "magic" number of 800px; any window width smaller than that results in it being reported as 800px.

The [downstream issue](https://github.com/keepassxreboot/keepassxc/issues/8293) has a few further details, but little else. KeePassXC seems to reproduce the issue, but not *all* Qt applications. Let me know of any further information that can help debug this.