Add mouse enter/leave events

[bvssvni]

This is a requested feature when you are dragging something and the cursor moves outside the window. On Linux and Windows the window stops emitting mouse coordinates. When returning to the window this can lead to undesirable behavior.

See PistonDevelopers/piston#958 for discussion.

[bvssvni]

@cybergeek94 you mentioned that only leave events could be added in WinAPI. Is this right?

[abonander]

As far as I can tell, yes. There's the WM_MOUSELEAVE message but no corresponding WM_MOUSEENTER message.

It looks like a mouse-entered message is supposed to be implied when you start getting WM_MOUSEMOVE messages again after WM_MOUSELEAVE: http://blogs.msdn.com/b/oldnewthing/archive/2003/10/13/55279.aspx

[wolfiestyle]

On Linux, X11 generates both events natively: https://tronche.com/gui/x/xlib/events/window-entry-exit/

[elinorbgr]

Wayland does natively generate both as well: http://vberger.github.io/wayland-client-rs/wayland_client/wayland/seat/enum.WlPointerEvent.html

[andybarron]

I'm going to take a stab at this.

I'm thinking of adding a MouseLeft event and letting MouseMoved imply it re-entered on all platforms. MouseWindow(bool) is cleaner, but kind of a pain in Windows...

Thoughts?

[andybarron]

The more I think about it, the more I like leaving MouseLeft as its own event. It avoids getting two events (e.g. MouseWindow(true) and MouseMoved(x, y)) when the mouse enters the window.

[tomaka]

Closing for rust-windowing/winit#53