Jared Wein [:jaws] (please needinfo? me) Reporter
	Description • 11 years ago

Open the attached plaintext file and resize the browser window horizontally. The browser janks pretty bad as it tries to do the word wrap calculations.

A quick profiling run shows that most of the time is spent in layout::DoReflow.

I understand that doing word-wrap is inherently more expensive than no word-wrap, but there should be some kind of optimizations we can make here.

	David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)
	Comment 1 • 11 years ago

It seems like interruptible reflow ought to help here, unless there's a non-interruptible flush happening (worth checking) or something prevents interruptible reflow from kicking in.

	Jared Wein [:jaws] (please needinfo? me) Reporter
	Comment 2 • 11 years ago

I placed a breakpoint in PresShell::ProcessReflowCommands and resized the window. It looks like it's already using an interruptible reflow: http://screencast.com/t/8Ws5Ed80rM

Could I be missing something here?

	David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)
	Comment 3 • 11 years ago

As long as aInterruptible is *always* true.  Sometimes true isn't good enough; the jank could come from the one time it isn't.  Or it could come from not having something that would lead to actually useful interrupting in a plaintext document... which seems entirely possible as well.

	Jared Wein [:jaws] (please needinfo? me) Reporter
	Comment 4 • 11 years ago

There are two uninterruptible reflows coming from:

1) http://hg.mozilla.org/mozilla-central/diff/c3956cae4197/browser/base/content/browser.js
2) http://hg.mozilla.org/mozilla-central/annotate/8d09f003e087/browser/base/content/tabbrowser.xml#l3440

both of which are triggered on "resize" events.

Removing the calls to .clientWidth and .boxObject.width removes all instances of uninterruptible reflows, but there is still noticeable lag compared to the non-word-wrap view (especially when expanding horizontally).

	Jared Wein [:jaws] (please needinfo? me) Reporter
	Comment 5 • 11 years ago

I should note that both of those links in comment #4 probably only cause reflows for chrome, not content.

	Karl Dubost💡 :karlcow
	Comment 6 • 6 years ago

Tested in Firefox Nightly 63.0a1 (2018-08-16) (64-bit), the linked testcase, I don't see any jankiness.