Subject: bug#23510: Strange redisplay in global highlighted mode moving the scroll bar

From: "[email-masked]" <[email-masked]>
Date: Tue, 10 May 2016 21:31:26 +0200 (CEST)

Maybe this is not a bug but I want to flag for completeness..

The following should be reproducible with the following simple init.el:

$ cat init.el
;;
(global-hl-line-mode 1)

Now, in the scratch buffer, copy and paste many times its text (;; This 
buffer...) so that the scratch buffer has many lines, not all contained in the 
Emacs window/frame. Move the cursor on a line, more or less at the center of 
the window. The line is highlighted (in green, by default). Now move slowly 
up/down the scroll bar, then the highlighted line seems to flicker as if it 
were redrawn at (almost) each pixel movement of the scroll bar.

Is this to be expected? Other software with same feature do not show this 
"flickering" and the highlight moves synchronously with the text.

The effect is visible not only with vertical scroll bar but also with the 
horizontal.

I see this with my master builds on Windows, GNU/linux and OSX.

Angelo