servo: Merge #15481 - stylo: Note need to flush styles when we set dirty descendant bits (from heycam:flush-styles); r=bholley

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This is the Servo-side part of https://bugzilla.mozilla.org/show_bug.cgi?id=1331294, already reviewed by @bholley.

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `__` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #__ (github issue number if applicable).

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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Source-Repo: https://github.com/servo/servo
Source-Revision: 2771b24ab36fb7953780660c275ec295de479599

--HG--
extra : subtree_source : https%3A//hg.mozilla.org/projects/converted-servo-linear
extra : subtree_revision : 35eda8a03fbfb0e83da1b0d500eeaf67f48a004e

servo/components/style/gecko_bindings/bindings.rs

@@ -633,6 +633,10 @@ extern "C" {
 extern "C" {
     pub fn Gecko_UnsetNodeFlags(node: RawGeckoNodeBorrowed, flags: u32);
 }
+extern "C" {
+    pub fn Gecko_SetOwnerDocumentNeedsStyleFlush(element:
+                                                     RawGeckoElementBorrowed);
+}
 extern "C" {
     pub fn Gecko_GetStyleContext(node: RawGeckoNodeBorrowed,
                                  aPseudoTagOrNull: *mut nsIAtom)
@@ -1125,10 +1129,6 @@ extern "C" {
 extern "C" {
     pub fn Servo_Element_ClearData(node: RawGeckoElementBorrowed);
 }
-extern "C" {
-    pub fn Servo_Element_ShouldTraverse(node: RawGeckoElementBorrowed)
-     -> bool;
-}
 extern "C" {
     pub fn Servo_StyleSheet_Empty(parsing_mode: SheetParsingMode)
      -> RawServoStyleSheetStrong;
@@ -1472,7 +1472,7 @@ extern "C" {
 extern "C" {
     pub fn Servo_TraverseSubtree(root: RawGeckoElementBorrowed,
                                  set: RawServoStyleSetBorrowed,
-                                 root_behavior: TraversalRootBehavior);
+                                 root_behavior: TraversalRootBehavior) -> bool;
 }
 extern "C" {
     pub fn Servo_AssertTreeIsClean(root: RawGeckoElementBorrowed);

servo/ports/geckolib/glue.rs

@@ -28,6 +28,7 @@ use style::gecko::selector_parser::{SelectorImpl, PseudoElement};
 use style::gecko::traversal::RecalcStyleOnly;
 use style::gecko::wrapper::DUMMY_BASE_URL;
 use style::gecko::wrapper::GeckoElement;
+use style::gecko_bindings::bindings;
 use style::gecko_bindings::bindings::{RawServoDeclarationBlockBorrowed, RawServoDeclarationBlockStrong};
 use style::gecko_bindings::bindings::{RawServoStyleRuleBorrowed, RawServoStyleRuleStrong};
 use style::gecko_bindings::bindings::{RawServoStyleSetBorrowed, RawServoStyleSetOwned};
@@ -163,14 +164,19 @@ fn traverse_subtree(element: GeckoElement, raw_data: RawServoStyleSetBorrowed,
     }
 }
 
+/// Traverses the subtree rooted at `root` for restyling.  Returns whether a
+/// Gecko post-traversal (to perform lazy frame construction, or consume any
+/// RestyleData, or drop any ElementData) is required.
 #[no_mangle]
 pub extern "C" fn Servo_TraverseSubtree(root: RawGeckoElementBorrowed,
                                         raw_data: RawServoStyleSetBorrowed,
-                                        behavior: structs::TraversalRootBehavior) -> () {
+                                        behavior: structs::TraversalRootBehavior) -> bool {
     let element = GeckoElement(root);
     debug!("Servo_TraverseSubtree: {:?}", element);
     traverse_subtree(element, raw_data,
                      behavior == structs::TraversalRootBehavior::UnstyledChildrenOnly);
+
+    element.has_dirty_descendants() || element.mutate_data().unwrap().has_restyle()
 }
 
 #[no_mangle]
@@ -341,24 +347,6 @@ pub extern "C" fn Servo_Element_ClearData(element: RawGeckoElementBorrowed) -> (
     GeckoElement(element).clear_data();
 }
 
-#[no_mangle]
-pub extern "C" fn Servo_Element_ShouldTraverse(element: RawGeckoElementBorrowed) -> bool {
-    let element = GeckoElement(element);
-    debug_assert!(!element.has_dirty_descendants(),
-                  "only call Servo_Element_ShouldTraverse if you know the element \
-                   does not have dirty descendants");
-    let result = match element.borrow_data() {
-        // Note that we check for has_restyle here rather than has_current_styles,
-        // because we also want the traversal code to trigger if there's restyle
-        // damage. We really only need the Gecko post-traversal in that case, so
-        // the servo traversal will be a no-op, but it's cheap enough that we
-        // don't bother distinguishing the two cases.
-        Some(d) => !d.has_styles() || d.has_restyle(),
-        None => true,
-    };
-    result
-}
-
 #[no_mangle]
 pub extern "C" fn Servo_StyleSheet_Empty(mode: SheetParsingMode) -> RawServoStyleSheetStrong {
     let url = ServoUrl::parse("about:blank").unwrap();
@@ -1077,6 +1065,7 @@ unsafe fn maybe_restyle<'a>(data: &'a mut AtomicRefMut<ElementData>, element: Ge
         if curr.has_dirty_descendants() { break; }
         curr.set_dirty_descendants();
     }
+    bindings::Gecko_SetOwnerDocumentNeedsStyleFlush(element.0);
 
     // Ensure and return the RestyleData.
     Some(data.ensure_restyle())