1274819 - Facebook "reason" mlCompared.html page is really janky, if you re-enable their disabled box-shadow rule

Status: RESOLVED DUPLICATE of bug 1250947

(Core :: Graphics, defect)

Description

[Daniel Holbert [:dholbert] (PTO until 2/24)]

(This performance bug has actually already been fixed in Nightly, but I'm filing it anyway just for the record, since it's not obvious that the patch that fixed it was expected to affect performance.)

STR:
 1) Visit https://facebook.github.io/reason/mlCompared.html

 2) Open inspector and find div with class="content-root"

 3) Check the (unchecked-by-default) checkbox for the "box-shadow" rule.
(It's unchecked to reflect that it's in the CSS but commented-out.)

 4) Try to scroll. (e.g. with page-up / page-down keys)

ACTUAL RESULTS:
Extremely janky scrolling. (Like, 1/3 to 1/2 second will elapse between me pressing Page Down and the page responding.)

EXPECTED RESULTS:
Smooth/responsive scrolling.

I get ACTUAL RESULTS in Firefox Release (46).
I get EXPECTED RESULTS on Nightly.

Fix range:
https://hg.mozilla.org/integration/mozilla-inbound/pushloghtml?fromchange=4de1094b41b27db10a5b0e5683ba4272c505ee9c&tochange=98818a65c221dc220432d1c3da491f3aa97c1a81

--> Fixed by bug 1250947.

Comment 1

[Daniel Holbert [:dholbert] (PTO until 2/24)]

I'll just dupe this to bug 1250947 (which fixed it), but any discussion about this particular Facebook use-case can happen here, to keep bug 1250947 clean.

Comment 2

[Daniel Holbert [:dholbert] (PTO until 2/24)]

(Looks like bug 1271142 is similar -- high CPU usage at least -- and was fixed by (& duped to) the same fix.)