script: Implement the FontFaceSet DOM API (#32576)

Add a skeleton implementation of FontFaceSet interface with support
for resolving the `document.fonts.ready` Promise when the loading of
web fonts is completed.

This change exposes new failures in the web platform tests.

These were ERROR before the change because `document.fonts.ready` caused
a `ReferenceError` causing the tests to be aborted and they now FAIL:
- /css/CSS2/linebox/vertical-align-top-bottom-001.html
- /css/css-flexbox/flex-one-sets-flex-basis-to-zero-px.html
- /css/css-fonts/generic-family-keywords-001.html
- /css/css-fonts/math-script-level-and-math-style/math-script-level-004.tentative.html
- /css/css-fonts/math-script-level-and-math-style/math-script-level-002.tentative.html
- /css/css-text/text-autospace/text-autospace-ligature-001.html
- /css/css-values/calc-size/calc-size-width.tentative.html

These were TIMEOUT before the change because `document.fonts.ready` was
a ReferenceError and the tests were asynchronous (reftest-wait). These now
FAIL because the assertions are now executed after fonts are loaded:
- /css/css-fonts/matching/fixed-stretch-style-over-weight.html
- /css/css-fonts/matching/range-descriptor-reversed.html
- /css/css-fonts/matching/stretch-distance-over-weight-distance.html
- /css/css-fonts/matching/style-ranges-over-weight-direction.html
- /css/css-fonts/variations/variable-box-font.html
- /css/css-fonts/variations/variable-gpos-m2b.html
- /css/css-fonts/variations/variable-gsub.html
- /css/css-fonts/variations/variable-opsz-size-adjust.html
- /css/css-position/sticky/position-sticky-change-top.html
- /css/css-position/sticky/position-sticky-fixed-ancestor.html
- /css/css-position/sticky/position-sticky-flexbox.html
- /css/css-position/sticky/position-sticky-grid.html
- /css/css-position/sticky/position-sticky-inline.html
- /css/css-position/sticky/position-sticky-rendering.html
- /css/css-position/sticky/position-sticky-stacking-context.html
- /css/css-position/sticky/position-sticky-table-td-left.html
- /css/css-position/sticky/position-sticky-table-td-right.html
- /css/css-position/sticky/position-sticky-table-tfoot-bottom.html
- /css/css-position/sticky/position-sticky-table-th-right.html
- /css/css-position/sticky/position-sticky-table-thead-top.html
- /css/css-position/sticky/position-sticky-table-tr-bottom.html
- /css/css-position/sticky/position-sticky-table-tr-top.html
- /css/css-position/sticky/position-sticky-writing-modes.html
- /css/css-pseudo/marker-intrinsic-contribution-001.html
- /css/css-text/hyphens/hyphens-character.html

These tests now PASS due to this patch:
* FAIL -> PASS
  - /html/canvas/element/text/2d.text.draw.fill.maxWidth.fontface.html
  - /html/canvas/element/text/2d.text.measure.width.empty.html
* TIMEOUT -> PASS
  - /css/css-fonts/variations/font-descriptor-range-reversed.html
  - /css/css-fonts/variations/variable-opsz.html
  - /css/css-position/sticky/position-sticky-table-th-left.html
* ERROR -> PASS
  - /css/css-fonts/generic-family-keywords-002.html
  - /css/css-fonts/generic-family-keywords-003.html
* These two tests only PASS in Layout 2020:
  - /css/CSS2/positioning/inline-static-position-001.html
  - /css/cssom-view/getBoundingClientRect-empty-inline.html

These two tests have subtests that PASS intermittenttly:
- /fetch/metadata/generated/css-font-face.sub.tentative.html
- /css/css-fonts/generic-family-keywords-001.html

These tests are new TIMEOUTS that used to FAIL because
`documents.fonts.ready` was undefined:
- /resource-timing/TAO-match.html
- /resource-timing/content-type.html
- /resource-timing/nextHopProtocol-is-tao-protected.https.html

The failure in `/resize-observer/change-layout-in-error.html` could be
due to an issue in the ResizeObserver implementation that is now exposed
with this change, but this needs more investigation.

Signed-off-by: Mukilan Thiyagarajan <mukilan@igalia.com>
Co-authored-by: Martin Robinson <mrobinson@igalia.com>

components/script/script_thread.rs

@@ -2125,7 +2125,7 @@ impl ScriptThread {
                 FocusIFrame(id, ..) => Some(id),
                 WebDriverScriptCommand(id, ..) => Some(id),
                 TickAllAnimations(id, ..) => Some(id),
-                WebFontLoaded(id) => Some(id),
+                WebFontLoaded(id, ..) => Some(id),
                 DispatchIFrameLoadEvent {
                     target: _,
                     parent: id,
@@ -2325,8 +2325,8 @@ impl ScriptThread {
             ConstellationControlMsg::WebDriverScriptCommand(pipeline_id, msg) => {
                 self.handle_webdriver_msg(pipeline_id, msg)
             },
-            ConstellationControlMsg::WebFontLoaded(pipeline_id) => {
-                self.handle_web_font_loaded(pipeline_id)
+            ConstellationControlMsg::WebFontLoaded(pipeline_id, success) => {
+                self.handle_web_font_loaded(pipeline_id, success)
             },
             ConstellationControlMsg::DispatchIFrameLoadEvent {
                 target: browsing_context_id,
@@ -3301,7 +3301,7 @@ impl ScriptThread {
     }
 
     /// Handles a Web font being loaded. Does nothing if the page no longer exists.
-    fn handle_web_font_loaded(&self, pipeline_id: PipelineId) {
+    fn handle_web_font_loaded(&self, pipeline_id: PipelineId, _success: bool) {
         let Some(document) = self.documents.borrow().find_document(pipeline_id) else {
             warn!("Web font loaded in closed pipeline {}.", pipeline_id);
             return;
@@ -3310,6 +3310,12 @@ impl ScriptThread {
         // TODO: This should only dirty nodes that are waiting for a web font to finish loading!
         document.dirty_all_nodes();
         document.window().add_pending_reflow();
+
+        // This is required because the handlers added to the promise exposed at
+        // `document.fonts.ready` are run by the event loop only when it performs a microtask
+        // checkpoint. Without the call below, this never happens and the promise is 'stuck' waiting
+        // to be resolved until another event forces a microtask checkpoint.
+        self.rendering_opportunity(pipeline_id);
     }
 
     /// Handles a worklet being loaded. Does nothing if the page no longer exists.