Re: Rendering performace vs. line-spacing

[Herman, Geza]

On 2021-01-08 15:35, Eli Zaretskii wrote:
> If screen lines overlap with the offending font, then the difference
> could be justified, because redrawing one screen line would require
> redrawing the two adjacent screen lines.  I'm a bit surprised by the
> large performance hit, though, so maybe it's worth looking at the code
> which gets run in the slow case, to see maybe some unnecessary work we
> are doing in that case.
>
> Do you have very wide windows, per chance?  Does the problem becomes
> smaller if you make your frames and windows narrower?
Yes, my full screen emacs has a width of ~270. Sometimes I use this as a 
full screen window, sometimes I split it into two with 
split-window-horizontally.

And yes, the problem becomes smaller if I make the window narrower, or 
if the screen contains less characters. This problem is most visible at 
a particular part of my code which contains a larger table, with long 
lines (~160).

I disabled redrawing caused by overlapping by commenting out the line 
which sets the "row->overlapping_p" flag (xdisp.c:21752), and the 
problem is gone away. But my fonts are clipped (not a surprise). But, it 
fixed the consolas case as well, and I don't see any clipping with this 
font. Maybe overlapping detection is too strict (I mean, emacs considers 
cases overlapping which are close to overlapping, but they aren't)? Or 
maybe I just don't notice the clipping.