Bug 110055 – Web Inspector: User Agent style stylesheet identifier keeps incrementing

• Status: NEW
• Component: Web Inspector

[Timothy Hatcher]

After calling CSSAgent.setRuleSelector for an author rule, the next time CSSAgent.getMatchedStylesForNode is called will cause all user agent styles to get a new stylesheet id, meaning a whole different CSSStyleId for those styles. All other styles and rules keep the same identifiers.

I expect the identifiers to stay constant when possible, especially for the immutable user agent stylesheet. Repeated calls to getMatchedStylesForNode without calling setRuleSelector does not change the stylesheet id for user agent styles.

I didn't test, but other stylesheet modification functions might also cause the user agent styles to change identifiers.

[Radar WebKit Bug Importer]

<rdar://problem/19281651>

[Blaze Burg]

Does this have a user impact, or is it just refactoring?

[Nikita Vasilyev]

Re-discovering this bug cost me over an hour of extra debugging of bug 202065	REGRESSION(r243264): Web Inspector: Style pane doesn't update after toggling CSS class :/

Brian, there's no user impact.

[Qianlang Chen]

This is very likely the result of having to re-parse the entire style sheet's text after nearly every style edits from the frontend (except for editing specifically  a style rule's header, as of today, which we sidestepped having to re-parse; see https://github.com/WebKit/WebKit/blob/6277bccb19de5ebd68797f369f8ff24fc4417f2e/Source/WebCore/inspector/InspectorStyleSheet.cpp#L1116-L1119; hopefully we can change that and bypass having to re-parse whenever we can.)

There is actually quite a noticeable UI impact, except it's currently suppressed by skip marking changes in user agent style sheet IDs as significant changes. If we didn't do that then the style property currently being edited will lose focus after every keystroke due to the handling of significant changes, which is very bad UX.

I discovered the same happening to imported rules. When a style sheet A imports style sheet B, every edit to style sheet A from the style details panel also results in style sheet B getting a new ID. Unfortunately we don't have the same bypass-significant-changes mechanism so the bad UX shows up there (https://bugs.webkit.org/show_bug.cgi?id=271403).