Bug 1766082: Allow content to wait for parent screen transforms r=emilio

Content processes can provide screen coordinates in e.g. window objects and events without waiting for the proper client-to-screen transforms to be given to them from the parent process.  This poses a problem for tests that want to check the screen coordinates, so we add SpecialPowers.ContentTransformsReceived() to allow content processes to wait for these transforms.

Differential Revision: https://phabricator.services.mozilla.com/D144742

dom/interfaces/base/nsIBrowserChild.idl

@@ -27,6 +27,12 @@ interface nsIBrowserChild : nsISupports
 
   void remoteDropLinks(in Array<nsIDroppedLinkItem> links);
 
+  /**
+   * Resolved after content has received a PBrowser::ChildToParentMatrix.
+   */
+  [implicit_jscontext]
+  Promise contentTransformsReceived();
+
   readonly attribute uint64_t tabId;
 
   /*

dom/ipc/BrowserChild.cpp

@@ -539,6 +539,7 @@ NS_IMPL_CYCLE_COLLECTION_UNLINK_BEGIN(BrowserChild)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mWebNav)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mBrowsingContext)
   NS_IMPL_CYCLE_COLLECTION_UNLINK(mSessionStoreChild)
+  NS_IMPL_CYCLE_COLLECTION_UNLINK(mContentTransformPromise)
   NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_REFERENCE
 NS_IMPL_CYCLE_COLLECTION_UNLINK_END
 
@@ -548,6 +549,7 @@ NS_IMPL_CYCLE_COLLECTION_TRAVERSE_BEGIN(BrowserChild)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mWebNav)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mBrowsingContext)
   NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mSessionStoreChild)
+  NS_IMPL_CYCLE_COLLECTION_TRAVERSE(mContentTransformPromise)
 NS_IMPL_CYCLE_COLLECTION_TRAVERSE_END
 
 NS_IMPL_CYCLE_COLLECTION_TRACE_BEGIN(BrowserChild)
@@ -1300,6 +1302,11 @@ mozilla::ipc::IPCResult BrowserChild::RecvChildToParentMatrix(
   mTopLevelViewportVisibleRectInBrowserCoords =
       aTopLevelViewportVisibleRectInBrowserCoords;
 
+  if (mContentTransformPromise) {
+    mContentTransformPromise->MaybeResolveWithUndefined();
+    mContentTransformPromise = nullptr;
+  }
+
   // Trigger an intersection observation update since ancestor viewports
   // changed.
   if (RefPtr<Document> toplevelDoc = GetTopLevelDocument()) {
@@ -3849,6 +3856,28 @@ void BrowserChild::NotifyContentBlockingEvent(
   }
 }
 
+NS_IMETHODIMP
+BrowserChild::ContentTransformsReceived(JSContext* aCx,
+                                        dom::Promise** aPromise) {
+  auto* globalObject = xpc::CurrentNativeGlobal(aCx);
+  ErrorResult rv;
+  if (mChildToParentConversionMatrix) {
+    // Already received content transforms
+    RefPtr<Promise> promise =
+        Promise::CreateResolvedWithUndefined(globalObject, rv);
+    promise.forget(aPromise);
+    return rv.StealNSResult();
+  }
+
+  if (!mContentTransformPromise) {
+    mContentTransformPromise = Promise::Create(globalObject, rv);
+  }
+
+  MOZ_ASSERT(globalObject == mContentTransformPromise->GetGlobalObject());
+  NS_IF_ADDREF(*aPromise = mContentTransformPromise);
+  return rv.StealNSResult();
+}
+
 BrowserChildMessageManager::BrowserChildMessageManager(
     BrowserChild* aBrowserChild)
     : ContentFrameMessageManager(new nsFrameMessageManager(aBrowserChild)),

dom/ipc/BrowserChild.h

@@ -920,6 +920,9 @@ class BrowserChild final : public nsMessageManagerScriptExecutor,
   Maybe<bool> mWindowSupportsProtectedMedia;
 #endif
 
+  // If set, resolve when we receive ChildToParentMatrix.
+  RefPtr<dom::Promise> mContentTransformPromise;
+
   DISALLOW_EVIL_CONSTRUCTORS(BrowserChild);
 };
 

testing/specialpowers/content/SpecialPowersChild.jsm

@@ -2248,6 +2248,21 @@ class SpecialPowersChild extends JSWindowActorChild {
       wrapCallback
     );
   }
+
+  /* Content processes asynchronously receive child-to-parent transformations
+   * when they are launched.  Until they are received, screen coordinates
+   * reported to JS are wrong.  This is generally ok.  It behaves as if the
+   * user repositioned the window.  But if we want to test screen coordinates,
+   * we need to wait for the updated data.
+   */
+  contentTransformsReceived(win) {
+    try {
+      // throw if win is not a remote browser.
+      return win.docShell.browserChild.contentTransformsReceived();
+    } catch (e) {
+      return Promise.resolve();
+    }
+  }
 }
 
 SpecialPowersChild.prototype._proxiedObservers = {