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 8:51 pm, Jared Finder wrote:
> 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:
> 

And I'm fairly certain this is the proper fix.  If a window is live, 
then the AREA-OR-POS made by posn-at-x-y should never be nil, I believe:

--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -415,7 +415,7 @@ mouse-drag-line
  		(when (window-live-p (setq posn-window (posn-window start)))
  		  ;; Add left edge of `posn-window' to `position'.
  		  (setq position (+ (window-pixel-left posn-window) position))
-		  (unless (nth 1 start)
+		  (unless (posn-area start)
  		    ;; Add width of objects on the left of the text area to
  		    ;; `position'.
  		    (when (eq (window-current-scroll-bars posn-window) 'left)
@@ -494,9 +494,11 @@ 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.
+	       ;; or left- or right-margin prefix ...
  	       (define-key map [right-divider] map)
  	       (define-key map [bottom-divider] map)
+	       (define-key map [left-margin] map)
+	       (define-key map [right-margin] map)
  	       map)
  	     t (lambda () (setq track-mouse old-track-mouse)))))))

   -- MJF