style: Only override to default color in high-contrast / forced-color…

…s mode if inheriting from transparent.

That way elements inside links, form controls, etc have the right
contrast, even if the page overrides the color.

We can't do it when inheriting from transparent because we've already
forgotten about the "right" color to inherit, so the default color makes
sense. But that is a pretty unlikely edge case.

Differential Revision: https://phabricator.services.mozilla.com/D73069

components/style/properties/cascade.rs

@@ -384,10 +384,15 @@ fn tweak_when_ignoring_colors(
             if color.0.is_transparent() {
                 return;
             }
-            let color = builder.device.default_color();
-            declarations_to_apply_unless_overriden.push(
-                PropertyDeclaration::Color(specified::ColorPropertyValue(color.into()))
-            )
+            // If the inherited color would be transparent, but we would
+            // override this with a non-transparent color, then override it with
+            // the default color. Otherwise just let it inherit through.
+            if builder.get_parent_inherited_text().clone_color().alpha == 0 {
+                let color = builder.device.default_color();
+                declarations_to_apply_unless_overriden.push(
+                    PropertyDeclaration::Color(specified::ColorPropertyValue(color.into()))
+                )
+            }
         },
         // We honor url background-images if backplating.
         #[cfg(feature = "gecko")]