https://searchfox.org/mozilla-central/rev/033d45ca70ff32acf04286244644d19308c359d5/browser/themes/shared/urlbar-searchbar.inc.css#216 See bug 1260036 comment 15. The current selector is: :not(#pageActionSeparator):not([hidden]) ~ #pageActionSeparator This rule is re-evaluated with some (tiny) amount of processing each time an id or hidden attribute is changed on any node in the document. The :not(#pageActionSeparator) part seems useless as we have a single element with the id pageActionSeparator. Emilio told me that for any `hidden` attribute change we need to walk all the siblings of the element on the next flush (to see if any is `#pageActionSeparator`). By adding #page-action-buttons > at the beginning of the selector we can avoid the walk of sibling nodes for "hidden" attribute changes on nodes that are not children of the #page-action-buttons node. Without having seen this in a profile, I'm not going to pretend this is a performance bug, but fixing it seems to be an improvement anyway.

Comment on attachment 8995960 [details] [diff] [review] Patch Review of attachment 8995960 [details] [diff] [review]: ----------------------------------------------------------------- rs=me

Pushed by florian@queze.net: https://hg.mozilla.org/integration/mozilla-inbound/rev/3db40ff608eb improve the efficiency of the CSS selector showing pageActionSeparator, rs=Gijs.