Pointer surface focus not checked before button press

[chrisduerr]

This is basically just a low-effort copy-paste of the Sway issue I've opened. However I noticed identical behavior with Anvil so I thought someone might be interested in solving this for Anvil too.

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When opening a new subsurface under the mouse pointer, wl_pointer::enter is not sent until after the mouse is moved over the window or a button is pressed. As a result, the first button press still goes through the subsurface under the cursor to the previously focused surface.

A video should make the usecase behind this a little more clear:

tmp.mp4

While the pointer button presses aren't shown in the video, you can see that the two buttons overlap exactly while being on two different subsurfaces. Ideally quickly "spamming" left click should cause the browser to open a lot of tabs, however every time the focus switches between the subsurfaces the first button press is ignored.