Add logic for CursorEntered/CursorLeft on Windows

[Fredemus]

Necessary to fix vizia/vizia#407, which is a bug that locks up the GUI when you press down on a mouse button inside the window and release the button outside the window.

[robbert-vdh]

I think this can be implemented without any additional state tracking or heuristics.

Take a look at the documentation of TrackMouseEvent. You can use it to get WM_MOUSEHOVER events when the cursor enters the client area, and WM_MOUSELEAVE events when the cursor leaves the client area. You need to set both the TME_HOVER and the TME_LEAVE flags in the TRACKMOUSEEVENT struct for that to work, and you need to repeat the call any time you receiver either of the two window messages. So the idea is that you call it after creating the window, and you then repeat the exact same call when handling those two window messages.

[Fredemus]

I first tried to do it like that, but WM_MOUSEHOVER ends up firing every time the mouse moves inside the window. This way it only happens the first time the mouse enters the window, which lines up with how it works on x11.

edit: also looks like winit does it the same way as I: https://github.com/rust-windowing/winit/blob/ee0db52ac49d64b46c500ef31d7f5f5107ce871a/src/platform_impl/windows/event_loop.rs#L1443

[robbert-vdh]

In that case you'd be able to just only set TME_HOVER when initially creating the window and when handling WM_MOUSELEAVE, and only set TME_LEAVE when handling WM_MOUSEENTER, no? If that works that sounds like a better solution than approximating it by tracking mouse moves.

[Fredemus]

WM_MOUSEENTER

For some reason there is no WM_MOUSEENTER, only WM_MOUSEHOVER

[robbert-vdh]

Sorry yeah I meant WM_MOUSEHOVER.

[Fredemus]

Ah okay. For reasons I don't really understand just sending TrackMouseEvent with TME_HOVER in WM_MOUSELEAVE means the event never fires, no matter if I set either TME_HOVER or TME_LEAVE initially, or both. I also tried to change dwHoverTime in case the system default was weird. It seemingly has to happen in WM_MOUSEMOVE.

I digged up the documentation I based this on.

[robbert-vdh]

I don't know how to feel about the Win32 API for this, but I guess this is the way to do it. 😅

LGTM.