Document or unify some edge case behaviors between platforms with mouse pointers

[Random-Scientist]

Description

While testing for #4514 we discovered that every platform we tested behaves a bit differently in some cases.

Moved = PointerMoved, Presence = PointerEntered + PointerLeft, Drag = DragMoved + DragEntered + DragLeft, None = nothing.

Cells marked TODO still need testing. To test, cargo run --example window on your platform of choice, perform the experiment and note which winit events you see in the log.

Test Case	Windows	Linux (X11)	Linux(Wayland)	macOS
Mouseover with LMB held but empty cursor1	None	Presence (GNOME)	None (GNOME)	None
Mouseover while holding draggable2	Drag (TODO check for Presence/Moved)	TODO	TODO	Moved, Presence, Drag
Mouseover while unfocused3	Moved, Presence	Moved, Presence (GNOME)	Moved, Presence (GNOME)	Moved, Presence4

1.

• Press LMB on desktop/background (not a draggable). (should unfocus the window if it is focused)
• move pointer over window.
• move pointer off of window.
• release LMB.

2.

• Press LMB over a file while focused on the winit window, picking it up for dragging.
• move pointer over window.
• move pointer off of window.
• release LMB.

3.

• unfocus window, pointer off of window.
• move pointer over window.
• move pointer off window.

4.

on winit master with older macOS versions, only Presence is emitted here (due to AppKit treatment of trackingRect), but receiving movement as well is more consistent with other platforms, and #4514 changes the implementation to respect this behavior across all macOS versions.

Relevant platforms

No response

[Random-Scientist]

another inconsistency, on macOS when I drag a link from the Mail app over a winit window I receive Presence (where with an empty cursor i get None as noted above), but not Drag (presumably the drag handling fails silently but appkit or winit still emits a pointer event??)