Make tree pseudos not precomputed since they are not really anonymous boxes.

components/style/gecko/generated/pseudo_element_definition.rs

@@ -546,6 +546,26 @@ impl PseudoElement {
                  PseudoElement::Before | PseudoElement::After | PseudoElement::FirstLine | PseudoElement::FirstLetter)
     }
 
+    /// Whether this pseudo-element is tree pseudo-element.
+    #[inline]
+    pub fn is_tree_pseudo_element(&self) -> bool {
+        match *self {
+            PseudoElement::MozTreeColumn(..) => true,
+            PseudoElement::MozTreeRow(..) => true,
+            PseudoElement::MozTreeSeparator(..) => true,
+            PseudoElement::MozTreeCell(..) => true,
+            PseudoElement::MozTreeIndentation(..) => true,
+            PseudoElement::MozTreeLine(..) => true,
+            PseudoElement::MozTreeTwisty(..) => true,
+            PseudoElement::MozTreeImage(..) => true,
+            PseudoElement::MozTreeCellText(..) => true,
+            PseudoElement::MozTreeCheckbox(..) => true,
+            PseudoElement::MozTreeProgressmeter(..) => true,
+            PseudoElement::MozTreeDropFeedback(..) => true,
+            _ => false,
+        }
+    }
+
     /// Gets the flags associated to this pseudo-element, or 0 if it's an
     /// anonymous box.
     pub fn flags(&self) -> u32 {

components/style/gecko/pseudo_element.rs

@@ -46,7 +46,7 @@ impl PseudoElement {
             return PseudoElementCascadeType::Eager
         }
 
-        if self.is_anon_box() {
+        if self.is_precomputed() {
             return PseudoElementCascadeType::Precomputed
         }
 
@@ -137,7 +137,7 @@ impl PseudoElement {
     /// Whether this pseudo-element is precomputed.
     #[inline]
     pub fn is_precomputed(&self) -> bool {
-        self.is_anon_box()
+        self.is_anon_box() && !self.is_tree_pseudo_element()
     }
 
     /// Covert non-canonical pseudo-element to canonical one, and keep a

components/style/gecko/pseudo_element_definition.mako.rs

@@ -94,6 +94,17 @@ impl PseudoElement {
                  ${" | ".join(map(lambda name: "PseudoElement::{}".format(name), EAGER_PSEUDOS))})
     }
 
+    /// Whether this pseudo-element is tree pseudo-element.
+    #[inline]
+    pub fn is_tree_pseudo_element(&self) -> bool {
+        match *self {
+            % for pseudo in TREE_PSEUDOS:
+            ${pseudo_element_variant(pseudo)} => true,
+            % endfor
+            _ => false,
+        }
+    }
+
     /// Gets the flags associated to this pseudo-element, or 0 if it's an
     /// anonymous box.
     pub fn flags(&self) -> u32 {

components/style/stylist.rs

@@ -1983,13 +1983,8 @@ impl CascadeData {
 
                         let map = match selector.pseudo_element() {
                             Some(pseudo) if pseudo.is_precomputed() => {
-                                if !selector.is_universal() ||
+                                debug_assert!(selector.is_universal());
-                                   !matches!(origin, Origin::UserAgent) {
+                                debug_assert!(matches!(origin, Origin::UserAgent));
-                                    // ::-moz-tree selectors may appear in
-                                    // non-UA sheets (even though they never
-                                    // match).
-                                    continue;
-                                }
 
                                 precomputed_pseudo_element_decls
                                     .as_mut()