Bug 312074 – document.styleSheets and shadowRoot.styleSheets incorrectly include adopted style sheets

• Component: New Bugs
• Status: RESOLVED FIXED

[Ahmad Saleem]

Per the CSSOM spec (§6.2 CSS Style Sheet Collections), the "document or shadow root CSS style sheets" list should only
   contain:       
                                                                                                                        
  1. CSS style sheets created from HTTP Link headers                                                                    
  2. CSS style sheets associated with the DocumentOrShadowRoot (i.e., from <style> and <link> elements), in tree order
                                                                                                                        
  Adopted style sheets (from adoptedStyleSheets) are only included in the "final CSS style sheets" list, which is a     
  separate concept used for style resolution. The styleSheets attribute (StyleSheetList) should reflect the "document or
   shadow root CSS style sheets", not the "final CSS style sheets".                                                     
                  
  WebKit currently includes adopted style sheets in StyleSheetList, causing document.styleSheets and                    
  shadowRoot.styleSheets to report incorrect lengths and entries when adoptedStyleSheets is non-empty.
                                                                                                                        
  Steps to Reproduce:                                                                                                   
   
  <style id="sheet1">body { width: 50%; }</style>                                                                       
  <script>                                                                                                              
    const sheet = new CSSStyleSheet();                                                                                  
    document.adoptedStyleSheets = [sheet];                                                                              
    console.log(document.styleSheets.length); // Expected: 1, Actual: 2                                                 
  </script>                                                                                                             
                                                                                                                        
  Expected: document.styleSheets.length is 1 (only the <style> element sheet).                                          
                  
  Actual: document.styleSheets.length is 2 (includes the adopted sheet).                                                
                  
  Root Cause:                                                                                                           
                  
  In Source/WebCore/style/StyleScope.cpp, collectActiveStyleSheets() appends adopted style sheets to both sheets (used  
  for style resolution — correct) and styleSheetsForStyleSheetsList (backing the DOM-visible StyleSheetList —
  incorrect).                                                                                                           
                  
  Spec Reference: https://drafts.csswg.org/cssom/#css-style-sheet-collections                                           
   
  Failing WPT Tests:                                                                                                    
  - css/cssom/StyleSheetList-constructable.html
  - css/cssom/StyleSheetList-constructable-with-style-recalc.html                                                       
                                                                 
  ---                                                                                                                   
  This also affects shadowRoot.styleSheets since the same code path in StyleScope is shared by both Document and        
  ShadowRoot.

[Radar WebKit Bug Importer]

<rdar://problem/174583340>

[Ahmad Saleem]

Pull request: https://github.com/WebKit/WebKit/pull/62572

[EWS]

Committed 311074@main (2dd7e967bacc): <https://commits.webkit.org/311074@main>

Reviewed commits have been landed. Closing PR #62572 and removing active labels.

[Ahmad Saleem]

Submitted web-platform-tests pull request: https://github.com/web-platform-tests/wpt/pull/59255