Auto merge of #22121 - gterzian:remove_constellation_block_in_navigation, r=paulrouget

Remove sync constellation -> embedder communication

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- [ ] `./mach build -d` does not report any errors
- [ ] `./mach test-tidy` does not report any errors
- [ ] These changes fix #22042 (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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components/script/dom/htmliframeelement.rs

@@ -274,8 +274,13 @@ impl HTMLIFrameElement {
         );
 
         let pipeline_id = self.pipeline_id();
-        // If the initial `about:blank` page is the current page, load with replacement enabled.
-        let replace = pipeline_id.is_some() && pipeline_id == self.about_blank_pipeline_id.get();
+        // If the initial `about:blank` page is the current page, load with replacement enabled,
+        // see https://html.spec.whatwg.org/multipage/#the-iframe-element:about:blank-3
+        let is_about_blank =
+            pipeline_id.is_some() && pipeline_id == self.about_blank_pipeline_id.get();
+        // Replacement enabled also takes into account whether the document is "completely loaded",
+        // see https://html.spec.whatwg.org/multipage/#the-iframe-element:completely-loaded
+        let replace = is_about_blank || !document.is_completely_loaded();
         self.navigate_or_reload_child_browsing_context(
             Some(load_data),
             NavigationType::Regular,