Auto merge of #26758 - jdm:stacking-context-transform-zero, r=mrobinson

Don't create empty stacking contexts in display lists

A recent change to euclid exposed that our display lists can contain Rects that contain NaN values. These NaNs originate from creating stacking contexts with transforms that scale the horizontal or vertical dimensions to 0. WebRender isn't prepared to handle these, so we need to not produce these empty stacking contexts when building the display list.

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] These changes fix #26592 and fix #26590
- [x] There are tests for these changes

components/layout_2020/display_list/stacking_context.rs

@@ -447,6 +447,14 @@ impl Fragment {
                     return;
                 }
 
+                // If this fragment has a transform applied that makes it take up no spae
+                // then we don't need to create any stacking contexts for it.
+                let has_non_invertible_transform =
+                    fragment.has_non_invertible_transform(&containing_block_info.rect.to_untyped());
+                if has_non_invertible_transform {
+                    return;
+                }
+
                 fragment.build_stacking_context_tree(
                     fragment_ref,
                     builder,
@@ -775,6 +783,15 @@ impl BoxFragment {
         })
     }
 
+    /// Returns true if the given style contains a transform that is not invertible.
+    fn has_non_invertible_transform(&self, containing_block: &Rect<Length>) -> bool {
+        let list = &self.style.get_box().transform;
+        match list.to_transform_3d_matrix(Some(containing_block)) {
+            Ok(t) => !t.0.is_invertible(),
+            Err(_) => false,
+        }
+    }
+
     /// Returns the 4D matrix representing this fragment's transform.
     pub fn calculate_transform_matrix(
         &self,
@@ -783,6 +800,10 @@ impl BoxFragment {
         let list = &self.style.get_box().transform;
         let transform =
             LayoutTransform::from_untyped(&list.to_transform_3d_matrix(Some(&border_rect)).ok()?.0);
+        // WebRender will end up dividing by the scale value of this transform, so we
+        // want to ensure we don't feed it a divisor of 0.
+        assert_ne!(transform.m11, 0.);
+        assert_ne!(transform.m22, 0.);
 
         let transform_origin = &self.style.get_box().transform_origin;
         let transform_origin_x = transform_origin