constellation: Store pending viewport changes (#39447)

It is possible for viewport changes to be received before the
constellation adds a browsing context to its managed set when
navigating. When that happens, the viewport change is ignored, and eg.
the hidpi will keep the default 1.0 value.
On later page navigation, this value is not updated and leads to
incorrect display zooming. The initial page load is fine in the current
embedders because they update the compositor and webview renderer even
if the constellation is not fully ready.

Testing: Testing is difficult because it's device dependent, but I
reproduce 100% of the time running winit_minimal on a Pinephone Pro.

Signed-off-by: webbeef <me@webbeef.org>

components/constellation/constellation.rs

@@ -490,6 +490,10 @@ pub struct Constellation<STF, SWF> {
 
     /// The image cache for the single-process mode
     image_cache: Box<dyn ImageCache>,
+
+    /// Pending viewport changes for browsing contexts that are not
+    /// yet known to the constellation.
+    pending_viewport_changes: HashMap<BrowsingContextId, ViewportDetails>,
 }
 
 /// State needed to construct a constellation.
@@ -743,6 +747,7 @@ where
                         state.compositor_proxy.cross_process_compositor_api,
                         rippy_data,
                     )),
+                    pending_viewport_changes: Default::default(),
                 };
 
                 constellation.run();
@@ -1164,6 +1169,12 @@ where
                 return;
             },
         };
+
+        // Override the viewport details if we have a pending change for that browsing context.
+        let viewport_details = self
+            .pending_viewport_changes
+            .remove(&browsing_context_id)
+            .unwrap_or(viewport_details);
         let browsing_context = BrowsingContext::new(
             bc_group_id,
             browsing_context_id,
@@ -5036,7 +5047,7 @@ where
     ) {
         debug!(
             "handle_change_viewport_details_msg: {:?}",
-            new_viewport_details.size.to_untyped()
+            new_viewport_details
         );
 
         let browsing_context_id = BrowsingContextId::from(webview_id);
@@ -5222,6 +5233,9 @@ where
                         ));
                 }
             }
+        } else {
+            self.pending_viewport_changes
+                .insert(browsing_context_id, new_viewport_details);
         }
 
         // Send resize message to any pending pipelines that aren't loaded yet.
@@ -5306,6 +5320,8 @@ where
             exit_mode,
         );
 
+        let _ = self.pending_viewport_changes.remove(&browsing_context_id);
+
         let browsing_context = match self.browsing_contexts.remove(&browsing_context_id) {
             Some(ctx) => ctx,
             None => {