bug#23598: 25.0.94; linum

[Eli Zaretskii]

> Date: Sun, 22 May 2016 08:59:35 +0200
> From: freezer@posteo.de
> 
> 1. When I turn on line numbers (with linum-mode), scrolling a large org file 
> is very slow. This only happens when the org sections are folded and not fully
> expanded.

linum-mode needs to count lines each time you scroll through the file,
and it needs to update the display for all the lines visible in the
window.  When many lines are hidden, updating a window requires
traversing a lot of lines in the buffer.  For example, the sample file
you sent shows almost 3000 lines in a single window, which is 100
times more than an average window without hidden lines.  So yes, this
is slow.

I suggest to switch to nlinum-mode from ELPA, which us much faster in
these cases.

> 2. With some themes, the line numbers are also truncated. Example (theme
> monokai): http://i.imgur.com/gkY8FGn.png
> 
> Example org file:
> https://github.com/syl20bnr/spacemacs/blob/master/doc/DOCUMENTATION.org

The monokai theme seems to implicitly enlarge the face used for the
line numbers and change its font (because Org headers use an enlarged
variable-pitch font), but it doesn't update the 'linum' face with that
information.  By default, the face for the line numbers has its
attributes merged with those of the face used to display the text
line, and since the 'linum' face by default doesn't specify any size
or font attributes, they are taken from the Org header face.  So the
calculation of character size linum-mode does is incorrect, because
the 'linum' face's font is still defined to be the original size
before it was enlarged.  I think setting the 'linum' face's attributes
to set specific font family and height attributes should fix this
problem.  It could be argued that the monokai theme should do that: it
clearly caters to linum-mode, since it does set some attributes of the
'linum' face.