Auto merge of #17501 - jyc:Text-children_changed, r=<try>

Have CharacterData call children_changed on its parent when data is set.

**Can't run WPT on my computer, so pushing here to run tests 😢**

Have CharacterData.SetData call children_changed on its parent when
data is set (if it is a Text node) so that HTMLStyleElement parents can
re-parse. Add variant ChildrenMutation::Text for it to use as the
mutation.

This fixes an issue where an empty <style> element's data is set but the
style is not updated. An HTMLStyleElement parent re-parses in its
children_changed implementation.

<!-- Please describe your changes on the following line: -->

---
<!-- 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
- [X] These changes are part of a series to fix #17182 (github issue number if applicable).

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

<!-- Reviewable:start -->
---
This change is [<img src="https://reviewable.io/review_button.svg" height="34" align="absmiddle" alt="Reviewable"/>](https://reviewable.io/reviews/servo/servo/17501)
<!-- Reviewable:end -->

components/script/dom/characterdata.rs

@@ -6,6 +6,7 @@
 
 use dom::bindings::cell::DOMRefCell;
 use dom::bindings::codegen::Bindings::CharacterDataBinding::CharacterDataMethods;
+use dom::bindings::codegen::Bindings::NodeBinding::NodeBinding::NodeMethods;
 use dom::bindings::codegen::Bindings::ProcessingInstructionBinding::ProcessingInstructionMethods;
 use dom::bindings::codegen::InheritTypes::{CharacterDataTypeId, NodeTypeId};
 use dom::bindings::codegen::UnionTypes::NodeOrString;
@@ -16,9 +17,10 @@ use dom::bindings::str::DOMString;
 use dom::comment::Comment;
 use dom::document::Document;
 use dom::element::Element;
-use dom::node::{Node, NodeDamage};
+use dom::node::{ChildrenMutation, Node, NodeDamage};
 use dom::processinginstruction::ProcessingInstruction;
 use dom::text::Text;
+use dom::virtualmethods::vtable_for;
 use dom_struct::dom_struct;
 use servo_config::opts;
 use std::cell::Ref;
@@ -83,8 +85,18 @@ impl CharacterDataMethods for CharacterData {
         let new_length = data.encode_utf16().count() as u32;
         *self.data.borrow_mut() = data;
         self.content_changed();
-        let node = self.upcast::<Node>();
-        node.ranges().replace_code_units(node, 0, old_length, new_length);
+
+        // If this is a Text node, we might need to re-parse (say, if our parent
+        // is a <style> element.) We don't need to if this is a Comment or
+        // ProcessingInstruction.
+        if let Some(_) = self.downcast::<Text>() {
+            let node = self.upcast::<Node>();
+            node.ranges().replace_code_units(node, 0, old_length, new_length);
+            if let Some(parent_node) = node.GetParentNode() {
+                let mutation = ChildrenMutation::Text;
+                vtable_for(&parent_node).children_changed(&mutation);
+            }
+        }
     }
 
     // https://dom.spec.whatwg.org/#dom-characterdata-length

components/script/dom/node.rs

@@ -2530,6 +2530,11 @@ pub enum ChildrenMutation<'a> {
         next: Option<&'a Node>,
     },
     ReplaceAll { removed: &'a [&'a Node], added: &'a [&'a Node] },
+    /// Mutation for when a Text node's data is modified.
+    /// This doesn't change the structure of the list, which is what the other
+    /// variants' fields are stored for at the moment, so this can just have no
+    /// arguments.
+    Text,
 }
 
 impl<'a> ChildrenMutation<'a> {
@@ -2581,13 +2586,17 @@ impl<'a> ChildrenMutation<'a> {
     }
 
     /// Get the child that follows the added or removed children.
+    /// Currently only used when this mutation might force us to
+    /// restyle later children (see HAS_SLOW_SELECTOR_LATER_SIBLINGS and
+    /// Element's implementation of VirtualMethods::children_changed).
     pub fn next_child(&self) -> Option<&Node> {
         match *self {
             ChildrenMutation::Append { .. } => None,
             ChildrenMutation::Insert { next, .. } => Some(next),
             ChildrenMutation::Prepend { next, .. } => Some(next),
             ChildrenMutation::Replace { next, .. } => next,
             ChildrenMutation::ReplaceAll { .. } => None,
+            ChildrenMutation::Text => None,
         }
     }
 
@@ -2625,6 +2634,7 @@ impl<'a> ChildrenMutation<'a> {
 
             ChildrenMutation::Replace { prev: None, next: None, .. } => unreachable!(),
             ChildrenMutation::ReplaceAll { .. } => None,
+            ChildrenMutation::Text => None,
         }
     }
 }

components/script/dom/nodelist.rs

@@ -279,6 +279,7 @@ impl ChildrenList {
                     self.last_index.set(middle as u32);
                 }
             },
+            ChildrenMutation::Text => {},
         }
     }