Reframe less on restyles that involve nonexistent first-letter frames in Gecko.

Gecko bug 1324618 part 9 servo bits: https://bugzilla.mozilla.org/show_bug.cgi?id=1324618

components/style/gecko/pseudo_element.rs

@@ -77,6 +77,12 @@ impl PseudoElement {
         matches!(*self, PseudoElement::Before | PseudoElement::After)
     }
 
+    /// Whether this pseudo-element is ::first-letter.
+    #[inline]
+    pub fn is_first_letter(&self) -> bool {
+        *self == PseudoElement::FirstLetter
+    }
+
     /// Whether this pseudo-element is lazily-cascaded.
     #[inline]
     pub fn is_lazy(&self) -> bool {

components/style/matching.rs

@@ -1507,9 +1507,25 @@ pub trait MatchMethods : TElement {
                 // triggering any change.
                 return StyleDifference::new(RestyleDamage::empty(), StyleChange::Unchanged)
             }
+            // FIXME(bz): This will keep reframing replaced elements.  Do we
+            // need this at all?  Seems like if we add/remove ::before or
+            // ::after styles we would get reframed over in match_pseudos and if
+            // that part didn't change and we had no frame for the
+            // ::before/::after then we don't care.  Need to double-check that
+            // we handle the "content" and "display" properties changing
+            // correctly, though.
+            // https://bugzilla.mozilla.org/show_bug.cgi?id=1376352
             return StyleDifference::new(RestyleDamage::reconstruct(), StyleChange::Changed)
         }
 
+        if pseudo.map_or(false, |p| p.is_first_letter()) {
+            // No one cares about this pseudo, and we've checked above that
+            // we're not switching from a "cares" to a "doesn't care" state
+            // or vice versa.
+            return StyleDifference::new(RestyleDamage::empty(),
+                                        StyleChange::Unchanged)
+        }
+
         // Something else. Be conservative for now.
         warn!("Reframing due to lack of old style source: {:?}, pseudo: {:?}",
                self, pseudo);

components/style/servo/selector_parser.rs

@@ -38,6 +38,7 @@ pub enum PseudoElement {
     After = 0,
     Before,
     Selection,
+    // If/when :first-letter is added, update is_first_letter accordingly.
     // Non-eager pseudos.
     DetailsSummary,
     DetailsContent,
@@ -110,6 +111,12 @@ impl PseudoElement {
         matches!(*self, PseudoElement::After | PseudoElement::Before)
     }
 
+    /// Whether the current pseudo element is :first-letter
+    #[inline]
+    pub fn is_first_letter(&self) -> bool {
+        false
+    }
+
     /// Whether this pseudo-element is eagerly-cascaded.
     #[inline]
     pub fn is_eager(&self) -> bool {