style: Invalidate style for ::selection.

This patch invalidates the style for `::selection`, which will restore the
behavior before the regression.

However, it's still not quite correct, because repaint is not triggered. Given
that `::selection` requires some major change to implement
https://github.com/w3c/csswg-drafts/issues/2474, we can address this problem
later.

Differential Revision: https://phabricator.services.mozilla.com/D35305

components/style/gecko/pseudo_element.rs

@@ -116,6 +116,12 @@ impl PseudoElement {
         *self == PseudoElement::Marker
     }
 
+    /// Whether this pseudo-element is the ::selection pseudo.
+    #[inline]
+    pub fn is_selection(&self) -> bool {
+        *self == PseudoElement::Selection
+    }
+
     /// Whether this pseudo-element is ::first-letter.
     #[inline]
     pub fn is_first_letter(&self) -> bool {

components/style/invalidation/element/invalidator.rs

@@ -805,6 +805,19 @@ where
                         if pseudo.is_marker() && self.element.marker_pseudo_element().is_none() {
                             invalidated_self = true;
                         }
+
+                        // FIXME: ::selection doesn't generate elements, so the
+                        // regular invalidation doesn't work for it. We store
+                        // the cached selection style holding off the originating
+                        // element, so we need to restyle it in order to invalidate
+                        // it. This is still not quite correct, since nothing
+                        // triggers a repaint necessarily, but matches old Gecko
+                        // behavior, and the ::selection implementation needs to
+                        // change significantly anyway to implement
+                        // https://github.com/w3c/csswg-drafts/issues/2474.
+                        if pseudo.is_selection() {
+                            invalidated_self = true;
+                        }
                     }
                 }
 

components/style/servo/selector_parser.rs

@@ -143,6 +143,12 @@ impl PseudoElement {
         false
     }
 
+    /// Whether this pseudo-element is the ::selection pseudo.
+    #[inline]
+    pub fn is_selection(&self) -> bool {
+        *self == PseudoElement::Selection
+    }
+
     /// Whether this pseudo-element is the ::before pseudo.
     #[inline]
     pub fn is_before(&self) -> bool {