bug#19988: 25.0.50; Drag events ending in different frame

[Stefan Monnier]

>> If one of the two is the top window (i.e. the one that's actually
>> visible), then we should use that one,
> Detecting whether "one of the two is the top window (i.e. the one that's
> actually visible)" requires access to the Z order of windows.

Not exactly: since this should be done by the C code, it can ask the
window-system, which is the "current window under the mouse pointer".
The result depends on Z ordering, but Emacs doesn't need to know that
Z ordering.

> The event structure can only contain what we put into it.  Which window
> would you put into this structure after you leave the frame containing
> the window where the start event occurred?

The GUI window that's under the mouse pointer (according to the
window-system).

>> If not, then it doesn't really matter if we "get it right" or not.
> If one frame obscures another, the obscured frame should IMHO not be
> considered a valid target.

If they're both obscured by a third window (which does not belong to
us), then I don't think it matters much either way.


        Stefan