[Webkit-unassigned] [Bug 195628] New: CSS Variable expression re-evaluated on dom change causing image reload

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=195628

            Bug ID: 195628
           Summary: CSS Variable expression re-evaluated on dom change
                    causing image reload
           Product: WebKit
           Version: Safari 12
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: CSS
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: malsup at salesforce.com

Created attachment 364397

  --> https://bugs.webkit.org/attachment.cgi?id=364397&action=review

demonstrates issue as well as source html

Summary:
It appears that Safari is recalculating CSS variables and updating unrelated DOM nodes any time the DOM is changed.  I have created a tiny test page that includes a :hover rule on the body element and a CSS variable for a div which assigns a background image.  Every time I trigger the body's :hover rule it seems to recalculate and update the div's background image (even though the resulting value is unchanged.)  In the case where the image is not cacheable this causes the image to reload every single time the :hover rule activates/deactivates.

Expected Results:
CSS variable evaluation should not have any side effects if the calculated value is unchanged.

Actual Results:
In my example, uncached images are reloaded.

This also happens on iPad (and presumably iPhone).

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