bug#10275: speedbar highlights first selected item in speedbar-highlight-face

[martin rudalics <rudalics@gmx.at>]

 > emacs -Q -f speedbar
 > ;; then select any source file in speedbar, click to unfold it, and
 > click over any variable or function.
 >
 > What happens:
 > - The file is highlighted in the Speedbar frame, with `speedbar-highlight-face'.
 > - The file is loaded into a buffer and displayed in a window.
 > - The selected variable or function is shown, and also highlighted
 > with `speedbar-highlight-face'.
 >
 > Clicking into another subitem (or even the same one) of the same file
 > does not bring back the highlighting. But clicking it into another
 > file's subitem does. It happens every time a new file is selected,
 > even if previously selected.
 >
 > I'm not sure this is a bug, because the behavior already happens in
 > 22.1, but certainly is unexpected, and the docstring of
 > `speedbar-highlight-face' talks only about buttons, not the source
 > file.

What happens is that after the mouse click speedbar does highlight the
line via `speedbar-highlight-one-tag-line' which installs in
`pre-command-hook' the function `speedbar-unhighlight-one-tag-line'.
Next comes a switch-frame event to select the frame of the buffer where
the highlight is and executing the corresponding `handle-switch-frame'
command triggers the `pre-command-hook' which swallows the highlight
immediately.

An easy work-around is

(defun speedbar-unhighlight-one-tag-line ()
   "Unhighlight the currently highlighted line."
   (when (and speedbar-highlight-one-tag-line
	     (not (eq this-command 'handle-switch-frame)))
     (speedbar-delete-overlay speedbar-highlight-one-tag-line)
     (setq speedbar-highlight-one-tag-line nil)
     (remove-hook 'pre-command-hook 'speedbar-unhighlight-one-tag-line)))

but I have no idea what Eric intended to accomplish in the first place
and delving into the code of speedbar or dframe is no fun at all.

martin