Auto merge of #25515 - pshaughn:checkpoints, r=jdm

Add microtask checkpoints to script elements and custom elements

Servo had a microtask checkpoint at the end of running a script, but there was also supposed to be one at the end of HTML-parsing a script element before Javascript-parsing the script itself, and there were supposed to be checkpoints immediately after the call to a custom element constructor. This adds those, passing all cases of one WPT test file.

---
<!-- 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 fix #25016 except for the remaining not-really-about-microtasks case #25514

<!-- Either: -->
- [X] There are tests for these changes

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components/script/dom/customelementregistry.rs

@@ -20,6 +20,7 @@ use crate::dom::bindings::error::{
 use crate::dom::bindings::inheritance::Castable;
 use crate::dom::bindings::reflector::{reflect_dom_object, DomObject, Reflector};
 use crate::dom::bindings::root::{Dom, DomRoot};
+use crate::dom::bindings::settings_stack::is_execution_stack_empty;
 use crate::dom::bindings::str::DOMString;
 use crate::dom::document::Document;
 use crate::dom::domexception::{DOMErrorName, DOMException};
@@ -543,6 +544,14 @@ impl CustomElementDefinition {
             }
         }
 
+        // https://heycam.github.io/webidl/#construct-a-callback-function
+        // https://html.spec.whatwg.org/multipage/#clean-up-after-running-script
+        if is_execution_stack_empty() {
+            window
+                .upcast::<GlobalScope>()
+                .perform_a_microtask_checkpoint();
+        }
+
         rooted!(in(*cx) let element_val = ObjectValue(element.get()));
         let element: DomRoot<Element> =
             match unsafe { DomRoot::from_jsval(*cx, element_val.handle(), ()) } {
@@ -690,6 +699,15 @@ fn run_upgrade_constructor(
         } {
             return Err(Error::JSFailed);
         }
+
+        // https://heycam.github.io/webidl/#construct-a-callback-function
+        // https://html.spec.whatwg.org/multipage/#clean-up-after-running-script
+        if is_execution_stack_empty() {
+            window
+                .upcast::<GlobalScope>()
+                .perform_a_microtask_checkpoint();
+        }
+
         // Step 8.3
         let mut same = false;
         rooted!(in(*cx) let construct_result_val = ObjectValue(construct_result.get()));