Re: Temporarily select-window, without updating mode-line face and cursor fill?

[JD Smith <jdtsmith@gmail.com>]

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> On May 1, 2021, at 3:31 PM, Eli Zaretskii <eliz@gnu.org> wrote:
> 
>> From: JD Smith <jdtsmith@gmail.com>
>> Date: Sat, 1 May 2021 14:46:31 -0400
>> 
>> I’m working on a small “in mode-line” scroll bar, and have implemented mouse events for it, which are working well.  When I interact using the mouse with the mode line of a non-selected window, I need to have that window temporarily selected, so I can calculate its line offsets and move within it.  But (just as for regular scrollbars), I’d like to retain the originally active window afterwards.
> 
> Please explain in more detail why do you think you need to select the
> window.  What are those "line offsets", and why "move within it"
> requires to select the window?

Sure, thanks.  I compute line positions at a number of points (window-start, window-end, and point-max) inside a mode line :eval form.  To compute line numbers at these positions, for speed in very long files, I’m actually using:

(defun mlscroll-fast-line-number-at-pos (pos)
  (save-excursion
    (goto-char pos)
    (string-to-number (format-mode-line "%l"))))

The operative bits of the “move within” look like:

   (when (/= targ start)
      (forward-line (- targ start))
      (recenter))

During normal automatic mode-line update, the selected window is automatically bound correctly while the mode line string is computed (without updating the mode line face, I might add!).  But during the mouse event callback, I must use the starting window of the mouse event (a press) as the window to target, since the user could click on any of them. 

I believe I can re-task all of the “computing lines” code to take a window argument, i.e. to avoid actually selecting the window itself.  But less clear is how to move forward and back a given number of lines (forward-line) and recenter in an unselected window, without selecting it temporarily and causing the mode line face flashing.


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