bug#61022: 28.2.50; Mouse tracking of high coordinates not working in rxvt-unicode

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

On 2023-01-23 5:15 am, Eli Zaretskii wrote:
>> From: git@vladimir.panteleev.md
>> Date: Sun, 22 Jan 2023 19:36:50 +0000
>> 
>> For some reason, xt-mouse with xterm-mouse-utf-8 on isn't working for 
>> me
>> under rxvt-unicode. I assume it worked at some point, but it's not
>> working now.
>> 
...
>> Reproducer:
>> 
>> - Run urxvt
>> - In it, run emacs -Q -nw
>> - M-: (require 'xt-mouse)
>> - M-: (setq xterm-mouse-utf-8 t)
>> - M-x xterm-mouse-mode
>> - Make sure the window is wider than 96 lines or columns
>> - Move the mouse around past the 96th line or column
>> 
>> Hope this helps.
> 
> Jared, any comments?
> 
> Thanks.

The change mostly works as inherit-input-method also causes UTF-8 
decoding to happen deep in read_char at the C level. (Is this 
intentional? I assume so because read-char just reads single bytes 
normally.) However, I think the following change is more appropriate:

-          (read-char nil nil 0.1))
+          ;; Read a character with input method conversion enabled
+          ;; but no conversion to force read-char to decode UTF-8
+          ;; byte sequences.
+          (let ((input-method-function nil))
+            (read-char nil t 0.1)))

This way we don't apply an actual input method conversion to characters. 
For example, without this additional change, if the 'british input 
method was active, the # ==> £ conversion would
happen, causing mouse events with X=2 to instead have X=131.

I verified this works for me locally.

   -- MJF