bug#36779: 25.1; mouse click not recognized for frames with large left position

[martin rudalics]

>> Can you mark text with the mouse?
> No, I cannot.

So the mouse down event is probably not recognized.

>> Does the bug happen only when the
>> entire frame is positioned to the right of 3840
> Yes, that's the case.
>
>> or do you see it with
>> a frame starting before 3840 but extending to the right of the display
>> as well?
> This situation does*not*  exhibit the bug.

Thanks for clarifying this.

> Thanks!  I built both Emacs 25.1 and 26.2 (latest release) from the
> sources, and confirmed that*both*  exhibits the same problem in my
> environment.
> I run gdb for the latter (Emacs 26.2)

Please continue using 26.2 for debugging.

> and the values of *dpyinfo and
> event->xbutton at that code point (line 8848) are:
>
> When the problem occurs:

These do not reveal any great differences, I suppose the
x_root values

>    x_root = 4326, y_root = 281, state = 0, button = 1, same_screen = 1}

and

>    x_root = 2467, y_root = 229, state = 0, button = 1, same_screen = 1}

are the expected ones, the first larger than 3840 the second smaller.

> I haven't yet pursued further executions (I need to sit in front of the
> second, 60-inch 4K display for this!), but will do (so, advice is welcome!).

The subsequent code is among the most convoluted ones Emacs has,
sprinkled with USE_GTK, USE_X_TOOLKIT and USE_TOOLKIT_SCROLL_BARS
defines, so you have all my sympathy.  But maybe stepping into
x_window_to_frame already exhibits that the latter is not able to find
a suitable frame ...

martin