patch 9.2.0880: scroll: window scrolls when using the autocommand window

patch 9.2.0880: scroll: window scrolls when using the autocommand window

Problem:  The window scrolls when an autocommand window is used while the
          cursor is behind multi-byte characters.
Solution: Use the byte column instead of the character count when computing
          how many screen lines the text up to the cursor takes.

fixes:  #12085
closes: #20884

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Signed-off-by: Hirohito Higashi <h.east.727@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>