[Webkit-unassigned] [Bug 126177] New: REGRESSION(r133214): Don't invalidate style when adding classes that don't match rules

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=126177

           Summary: REGRESSION(r133214): Don't invalidate style when
                    adding classes that don't match rules
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: Unspecified
        OS/Version: Unspecified
            Status: NEW
          Keywords: BlinkMergeCandidate
          Severity: Normal
          Priority: P2
         Component: CSS
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: rniwa at webkit.org
                CC: koivisto at iki.fi, akling at apple.com


Consider merging https://chromium.googlesource.com/chromium/blink/+/50ffb9fad31542e86de85e3305894d99b1d81d34
or come up with a better fix if the regression exists in WebKit.

r133214 caused a regression by making us mark elements for style recalc
when adding new classes that don't match any rules due to a logic error
in checkSelectorForClassChange. This patch corrects the logic error so
that now adding and removing class names that don't match rules will
not trigger a style recalc.

To facilitate testing this patch I added a new Internals API:
updateStyleAndReturnAffectedElementCount(). It updates the style of the document
and returns the number of elements that had a style computed for them. This
will also allow writing future tests when we make things smarter to avoid
recalcs.

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