1300879 - Slow restyle on large page due to viewport being resized

Status: RESOLVED FIXED

(Core :: CSS Parsing and Computation, defect, P3)

Description

[Markus Stange [:mstange]]

STR:
 1. Go to https://html.spec.whatwg.org/ and wait for it to load.
 2. Show / hide your bookmarks toolbar using View -> Toolbars -> Bookmarks Toolbar.
 3. Immediately after step 2, try to select text on the page.

The page hangs for about three seconds, doing restyling. The same happens when you open the findbar.

The restyle is triggered by nsPresContext::MediaFeatureValuesChanged.

Comment 1

[Boris Zbarsky [:bzbarsky]]

Is it slower than just restyling everything?  Because depending on what media rules are present, that may in fact be happening....

Comment 2

[Boris Zbarsky [:bzbarsky]]

And note that this particular page uses lots of viewport-relative units (vh and vw) last I looked, so changing the viewport height does in fact need to recalculate all sorts of stuff.  :(

Comment 3

[Markus Stange [:mstange]]

(In reply to Boris Zbarsky [:bz] from comment #1)
> Is it slower than just restyling everything?

I haven't checked, because I thought restyling everything was unnecessary. But it sounds like it is necessary. Oh well.

Comment 4

[Gregory Pappas [:gregp]]

The good news is that the style jank is gone, I did mozregression twice and am confident that bug 1718337 is what fixed the last of it. The bad news is that I'm getting 82s script jank, spun into bug 1771742