On 2016-12-22 11:25, Eli Zaretskii wrote:
> I indeed see slow redisplay with your test files (but only after
> changing 'append' to 'prepend', because otherwise Emacs selects a
> different font for the arrows).

I see, thanks; prepend is clearly not what I want, since I trying to set these fonts as a fallback.

> But when I looked at the display
> closely, I found that the arrows are not displayed using Segoe UI
> Emoji font, they are displayed by some other font.  In fact, the only
> characters in that test which use the Segoe UI Emoji font are blanks!
> Did you try using "C-u C-x =" to see which characters are displayed
> using the Emoji font in your case?

I did; append works fine here. Everything is displayed with XITS Math, as expected.  prepend uses Segoe or XITS for all text, which I don't want.  It's good for testing, though, since I guess that otherwise your Emacs picks other fonts that are available on your machine.

> I think that's because Segoe UI Emoji doesn't have glyphs for the
> arrows, at least on my system.  So I modified your 21028.fixed.el
> recipe as follows:

Thanks, but I'm not sure I see why you're suggesting this :/ If you see the slowdown, then it seems that managed to reproduce the problem, right?  Is there anything preventing us from applying the fix that you described last month?

The example only shows arrows because I tried to make it minimal; my bug report is about the fact that, when I set up Emacs to display characters the way I want them, it gets slow.  I readily admit that changing the font set-up may make the problem go away, but it's not a satisfactory solution (I still get support requests from users of my packages about degraded performance due to prettify-symbols-mode, and it's hard to tell them to use different fonts).

Recall that the original bug report didn't even use symbols: I first found out about this when trying to edit text written in Chinese.  I ran into the problem regularly in various settings since then, until I reverted the faulty commit (and later reapplied the safe part of it that you isolated).  Since then I've never run into this issue.  I've also confirmed on other users' laptops that reverting the faulty commit fixes the issue.

The fixes that you suggested to my minimal example do work around the issue in that case (another work around for me is to just remove the Segoe UI line), but these fixes are not applicable to my full font configuration.  I can post my full configuration if you want to suggest improvements that would work around the issue in my specific case (alternatively, it's available here: https://github.com/cpitclaudel/.emacs.d/blob/master/init/fonts.el), but this seems like a waste of your time: there's no doubt that others will run into the same issues (indeed, they already do), and fixing their configurations one by one isn't practical (assuming we even hear about them).

Cheers,
Clément.