Bug 1938619 - Add a pref (disabled by default) to enable intrinsic sizing of flexible grid tracks based on grid items that span multiple tracks. r=layout-grid-reviewers,dholbert

The pref layout.css.grid-flex-spanning-items-intrinsic-sizing.enabled is
expected to be removed once these regressions are fixed. This should be a good
compromise between maintaining the code from bug 1916849, and avoiding
breakage/extra backouts.

Differential Revision: https://phabricator.services.mozilla.com/D235089

layout/generic/nsGridContainerFrame.cpp

@@ -6814,6 +6814,10 @@ void nsGridContainerFrame::Tracks::ResolveIntrinsicSize(
         // Set eIsFlexing on the item state here to speed up
         // FindUsedFlexFraction later.
         gridItem.mState[mAxis] |= ItemState::eIsFlexing;
+        if (!StaticPrefs::
+                layout_css_grid_flex_spanning_items_intrinsic_sizing_enabled()) {
+          continue;
+        }
         items = &flexSpanningItems;
       }