bug#41156: margins interfere with xterm-mouse-mode

[Jared Finder via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

On 2021-06-14 5:56 am, Eli Zaretskii wrote:
>> From: Lars Ingebrigtsen <larsi@gnus.org>
>> Date: Mon, 14 Jun 2021 14:46:22 +0200
>> Cc: 41156@debbugs.gnu.org, Neil Okamoto <neil.okamoto@gmail.com>
>> 
>> martin rudalics <rudalics@gmx.at> writes:
>> 
>> > Unless you do
>> >
>> > (set-window-margins (selected-window) 2 0)
>> >
>> > in the window at right.
>> 
>> Right, so I guess xterm-mouse-mode needs to define a key binding in 
>> the
>> margin area for these mouse commands?
>> 
>> Hm...  Well, I took a quick peek at xt-mouse.el for the first time in 
>> my
>> life, and that doesn't seem to be how that mode works at all.  Is
>> anybody familiar enough with xt-mouse that it's obvious to them what's
>> going wrong in this case?
> 
> Jared, can you help us out here, please?

xterm-mouse-mode is running fine, it is correctly generating 
mouse-motion events with proper X,Y coordinates.

The actual drag keybinding is handled in mouse-drag-line in mouse.el.  
The following patch mostly works for me, though I see issues when 
dragging to the left and the left buffer has a margin of width greater 
than 1.  I think there's some incorrect logic in how the temporarily 
bound move function is converting calculating positions:

@@ -494,9 +494,12 @@ mouse-drag-line
  	       (define-key map [header-line] map)
  	       (define-key map [vertical-line] map)
  	       ;; ... and some maybe even with a right- or bottom-divider
-	       ;; prefix.
+	       ;; prefix ...
  	       (define-key map [right-divider] map)
  	       (define-key map [bottom-divider] map)
+               ;; ... and the margins too.
+               (define-key map [left-margin] map)
+               (define-key map [right-margin] map)
  	       map)
  	     t (lambda () (setq track-mouse old-track-mouse)))))))

I'll investigate a bit more later.

   -- MJF