By accessing |document.documentElement.clientWidth| and calling |.getBoundingClientRect()| for all hidden elements (+ 1) about:newtab causes uninterruptible reflows when the page is first shown. 1) Code that collects telemetry data should not have any performance impact. 2) There might be a different/better way to determine how many children of #newtab-grid are visible?

Looks like nsIDOMWindowUtils has everything we need.

Comment on attachment 8493854 [details] [diff] [review] 0001-Bug-1071635-Get-rid-of-uninterruptible-reflows-cause.patch Sorry, forgot to run tests. Will need to make some adjustments.

Passes tests now. Turns out I was actually calculating the wrong index by not ignoring empty cells.

Looks like this snuck in with bug 1042214. The changes to browser_tabopen_reflow.js which obviously caught this have unfortunately not been reviewed :/

(In reply to Tim Taubert [:ttaubert] from comment #4) > Looks like this snuck in with bug 1042214. The changes to > browser_tabopen_reflow.js which obviously caught this have unfortunately not > been reviewed :/ Sorry about that. I thought it was explicit that we wanted the reflow to calculate positions for the functionality, so the test was updated for that.

Yeah, I assumed this was a quick fix to unbreak the test but we don't actually want sync reflows :) I could maybe add something to the test that explicitly states what the test tries to prevent and who to ask for review? The test itself isn't that clear I think.