xwayland: map a held pointer through the monitor under the cursor

[donutinit]

I use a music player to manage my sound-effect collection, on i3wm I dragged and dropped files from the player to DaVinci Resolve. When I tried to do that on wayland Davinci's windows flickered strangely, the solution was to launch the music player in Xwayland; but it came with a catch, i could only drag and drop from the same monitor. With some thinkering I figured out that the problem was that Xwayland was treating the window/displays positioning different as the hyprland config.

I traced it to mouseMoveUnified in InputManager.cpp: while a button is held the pointer focus stays on the X11 window where the press started, and the coords sent to X were the cursor's offset from that window, scaled for that window's monitor. Xwayland lays out the monitors in its own space (a row in monitor order, with per-monitor scales under force_zero_scaling), so once the cursor crossed to another monitor X saw the pointer somewhere else, sometimes off-screen, and that's why dnd only worked on the same monitor.

Now when the cursor is outside the held window it gets mapped through the monitor it's actually on. For that I added globalToBuffer(pos, origin) to the window backends (WindowBackend.hpp, X11Backend.cpp, WaylandBackend.cpp): on X11 it converts the cursor and the window origin to Xwayland coords and takes the difference, on Wayland it's just pos - origin. Inside the window it works like before.

I also fixed the monitor lookup in waylandToXWaylandCoords (XWaylandManager.cpp): with force_zero_scaling it compared logical coords against each monitor's physical size, now it uses the logical size.

For testing I added xwaylandPointerFollowsCursorAcrossMonitors to hyprtester, with a plugin helper, hl.plugin.test.expect_xwayland_pointer. It creates two monitors in the opposite order of their position so Xwayland's layout doesn't match Hyprland's, opens xeyes on the right one, holds a click, moves to the left one and checks that X sees the pointer where the cursor really is, with and without force_zero_scaling.

Did you use AI in any way shape or form in the making of this PR? If so, detail how you used it. (Undisclosed AI use is a violation of the AI Policy and is punishable)

Yes, it helped me to understand how does wayland interact with xwayland. And it guided me A LOT in the steps of troubleshooting what was causing the problem. I am quite new to window managers and I fucking struggled a lot lmao

Is it ready for merging, or does it need work?

Ready to merge

[github-actions]

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