Fix an issue with hide_cursor and container focus

[tomekb234]

This small patch fixes an issue with how hide_cursor feature interacts with keyboard control and wl_pointer.{enter,leave} Wayland events. It seems to fix #6245.

When the cursor is hidden over a container which regains focus without mouse warping (for example with keyboard bindings for focus and workspace commands when mouse_warping is set to output), the Wayland event wl_pointer.enter is sent despite the cursor remaining hidden (and it is not reverted with wl_pointer.leave). This is noticeable and somewhat bothersome in particular when using a web browser primarily with a keyboard — when the browser regains focus and a web site is scrolled with a keyboard, some hyperlinks become unexpectedly underlined (sometimes causing popups to appear, as on Wikipedia) because of the invisible cursor hovering above them, and getting rid of this behavior (for as long as the browser remains constantly focused) requires moving the cursor only to hide it again and send the wl_pointer.leave event.

This bug can be observed more precisely with wev, for example with these steps:

1. Start sway with default configuration with additional line seat * hide_cursor 1000.
2. Run wev -f wl_pointer:enter -f wl_pointer:leave.
3. Move the cursor over the wev window and wait for it to hide — this should show enter and leave events in wev output.
4. Using only keyboard, switch to another workspace and then return to the one with wev — this should show an erroneous enter event in wev output.
5. Move the cursor without leaving the wev window and wait for it to hide — this should only show leave event in wev output.

The fix appears to be a simple check for cursor->hidden in handle_rebase (it works for hide_cursor when-typing as well), but honestly I haven't overanalyzed this input system and can't say for sure if this is the best solution.