LibWeb: Resolve content-visibility fixme in html details element

Author: https://github.com/EdwinHoksberg
Commit: 0ae048102c
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/267
Reviewed-by: https://github.com/AtkinsSJ ✅

Tests/LibWeb/Layout/expected/details-closed.txt

@@ -8,6 +8,7 @@ Viewport <#document> at (0,0) content-size 800x600 children: not-inline
             "I'm a summary"
         TextNode <#text>
         ListItemMarkerBox <(anonymous)> at (8,8) content-size 12x17 children: not-inline
+      BlockContainer <slot> at (8,25) content-size 784x0 children: not-inline
       BlockContainer <(anonymous)> at (8,25) content-size 784x0 children: inline
         TextNode <#text>
 
@@ -19,4 +20,5 @@ ViewportPaintable (Viewport<#document>) [0,0 800x600]
       PaintableWithLines (ListItemBox<SUMMARY>) [32,8 760x17]
         TextPaintable (TextNode<#text>)
         MarkerPaintable (ListItemMarkerBox(anonymous)) [8,8 12x17]
+      PaintableWithLines (BlockContainer<SLOT>) [8,25 784x0]
       PaintableWithLines (BlockContainer(anonymous)) [8,25 784x0]

Userland/Libraries/LibWeb/HTML/HTMLDetailsElement.cpp

@@ -185,9 +185,8 @@ void HTMLDetailsElement::update_shadow_tree_style()
             display: block;
         )~~~"_string));
     } else {
-        // FIXME: Should be `display: block` but we do not support `content-visibility: hidden`.
         MUST(m_descendants_slot->set_attribute(HTML::AttributeNames::style, R"~~~(
-            display: none;
+            display: block;
             content-visibility: hidden;
         )~~~"_string));
     }