style: Don't cache structs if they have been adjusted.

StyleAdjuster looks a bunch of things that we don't record as dependencies.

components/style/properties/properties.mako.rs

@@ -2602,6 +2602,11 @@ pub struct StyleBuilder<'a> {
     /// The pseudo-element this style will represent.
     pseudo: Option<<&'a PseudoElement>,
 
+    /// Whether we have mutated any reset structs since the the last time
+    /// `clear_modified_reset` was called.  This is used to tell whether the
+    /// `StyleAdjuster` did any work.
+    modified_reset: bool,
+
     /// The writing mode flags.
     ///
     /// TODO(emilio): Make private.
@@ -2665,6 +2670,7 @@ impl<'a> StyleBuilder<'a> {
             reset_style,
             pseudo,
             rules,
+            modified_reset: false,
             custom_properties,
             writing_mode,
             font_size_keyword,
@@ -2706,6 +2712,7 @@ impl<'a> StyleBuilder<'a> {
             inherited_style_ignoring_first_line: inherited_style,
             reset_style,
             pseudo,
+            modified_reset: false,
             rules: None, // FIXME(emilio): Dubious...
             custom_properties: style_to_derive_from.custom_properties(),
             writing_mode: style_to_derive_from.writing_mode,
@@ -2745,6 +2752,7 @@ impl<'a> StyleBuilder<'a> {
 
         % if not property.style_struct.inherited:
         self.flags.insert(::properties::computed_value_flags::INHERITS_RESET_STYLE);
+        self.modified_reset = true;
         % endif
 
         % if property.ident == "content":
@@ -2770,6 +2778,10 @@ impl<'a> StyleBuilder<'a> {
         let reset_struct =
             self.reset_style.get_${property.style_struct.name_lower}();
 
+        % if not property.style_struct.inherited:
+        self.modified_reset = true;
+        % endif
+
         self.${property.style_struct.ident}.mutate()
             .reset_${property.ident}(
                 reset_struct,
@@ -2786,6 +2798,10 @@ impl<'a> StyleBuilder<'a> {
         &mut self,
         value: longhands::${property.ident}::computed_value::T
     ) {
+        % if not property.style_struct.inherited:
+        self.modified_reset = true;
+        % endif
+
         <% props_need_device = ["content", "list_style_type", "font_variant_alternates"] %>
         self.${property.style_struct.ident}.mutate()
             .set_${property.ident}(
@@ -2849,11 +2865,17 @@ impl<'a> StyleBuilder<'a> {
 
         /// Gets a mutable view of the current `${style_struct.name}` style.
         pub fn mutate_${style_struct.name_lower}(&mut self) -> &mut style_structs::${style_struct.name} {
+            % if not property.style_struct.inherited:
+            self.modified_reset = true;
+            % endif
             self.${style_struct.ident}.mutate()
         }
 
         /// Gets a mutable view of the current `${style_struct.name}` style.
         pub fn take_${style_struct.name_lower}(&mut self) -> UniqueArc<style_structs::${style_struct.name}> {
+            % if not property.style_struct.inherited:
+            self.modified_reset = true;
+            % endif
             self.${style_struct.ident}.take()
         }
 
@@ -2902,6 +2924,16 @@ impl<'a> StyleBuilder<'a> {
                  longhands::_moz_top_layer::computed_value::T::top)
     }
 
+    /// Clears the "have any reset structs been modified" flag.
+    fn clear_modified_reset(&mut self) {
+        self.modified_reset = false;
+    }
+
+    /// Returns whether we have mutated any reset structs since the the last
+    /// time `clear_modified_reset` was called.
+    fn modified_reset(&self) -> bool {
+        self.modified_reset
+    }
 
     /// Turns this `StyleBuilder` into a proper `ComputedValues` instance.
     pub fn build(self) -> Arc<ComputedValues> {
@@ -3469,11 +3501,6 @@ where
 
     let mut builder = context.builder;
 
-    {
-        StyleAdjuster::new(&mut builder)
-            .adjust(layout_parent_style, flags);
-    }
-
     % if product == "gecko":
         if let Some(ref mut bg) = builder.get_background_if_mutated() {
             bg.fill_arrays();
@@ -3493,6 +3520,16 @@ where
         }
     % endif
 
+    builder.clear_modified_reset();
+
+    StyleAdjuster::new(&mut builder)
+        .adjust(layout_parent_style, flags);
+
+    if builder.modified_reset() {
+        context.rule_cache_conditions.borrow_mut()
+            .set_uncacheable();
+    }
+
     builder.build()
 }