Fixed backgrounds should not be affected by the transform, but should always render as though offset from the viewport.
Created attachment 28389 [details] simple example of problem
Seeing this in nightly build, too: 530.1+ Have attached a pretty simple example file. There's WebKit "transform:" rules and everywhere "left:" rules in the CSS to demonstrate the problem. (The "everywhere" section is commented out by default.)
Known issue.
Spec <http://dev.w3.org/csswg/css-transforms/> now says: "Fixed backgrounds on the root element are affected by any transform specified for that element. For all other elements that are effected by a transform (i.e. have a transform applied to them, or to any of their ancestor elements), a value of ‘fixed’ for the ‘background-attachment’ property is treated as if it had a value of ‘scroll’. The computed value of ‘background-attachment’ is not affected."
(In reply to comment #4) > Spec <http://dev.w3.org/csswg/css-transforms/> now says: > "Fixed backgrounds on the root element are affected by any transform > specified for that element. For all other elements that are effected by a > transform (i.e. have a transform applied to them, or to any of their > ancestor elements), a value of ‘fixed’ for the ‘background-attachment’ > property is treated as if it had a value of ‘scroll’. The computed value of > ‘background-attachment’ is not affected." We (Mozilla) have shipped this change in Firefox 48. IIRC Edge has also shipped it.