Bug 1575093 - Ensure initial-scale in a meta viewport tag is respected. r=hiro

Differential Revision: https://phabricator.services.mozilla.com/D42949

--HG--
extra : moz-landing-system : lando

gfx/layers/apz/test/gtest/mvm/TestMobileViewportManager.cpp

@@ -74,6 +74,7 @@ class MockMVMContext : public MVMContext {
     return Some(mDisplaySize);
   }
   bool AllowZoomingForDocument() const { return true; }
+  bool IsDocumentLoading() const { return false; }
 
   void SetResolutionAndScaleTo(float aResolution,
                                ResolutionChangeOrigin aOrigin) {

layout/base/GeckoMVMContext.cpp

@@ -124,6 +124,11 @@ bool GeckoMVMContext::AllowZoomingForDocument() const {
   return nsLayoutUtils::AllowZoomingForDocument(mDocument);
 }
 
+bool GeckoMVMContext::IsDocumentLoading() const {
+  MOZ_ASSERT(mDocument);
+  return mDocument->GetReadyStateEnum() == Document::READYSTATE_LOADING;
+}
+
 void GeckoMVMContext::SetResolutionAndScaleTo(float aResolution,
                                               ResolutionChangeOrigin aOrigin) {
   MOZ_ASSERT(mPresShell);

layout/base/GeckoMVMContext.h

@@ -47,6 +47,7 @@ class GeckoMVMContext : public MVMContext {
       const override;
   Maybe<LayoutDeviceIntSize> GetContentViewerSize() const override;
   bool AllowZoomingForDocument() const override;
+  bool IsDocumentLoading() const override;
 
   void SetResolutionAndScaleTo(float aResolution,
                                ResolutionChangeOrigin aOrigin) override;

layout/base/MVMContext.h

@@ -51,6 +51,7 @@ class MVMContext {
       const = 0;
   virtual Maybe<LayoutDeviceIntSize> GetContentViewerSize() const = 0;
   virtual bool AllowZoomingForDocument() const = 0;
+  virtual bool IsDocumentLoading() const = 0;
 
   virtual void SetResolutionAndScaleTo(float aResolution,
                                        ResolutionChangeOrigin aOrigin) = 0;

layout/base/MobileViewportManager.cpp

@@ -147,7 +147,23 @@ MobileViewportManager::HandleEvent(dom::Event* event) {
 
 void MobileViewportManager::HandleDOMMetaAdded() {
   MVM_LOG("%p: got a dom-meta-added event\n", this);
-  RefreshViewportSize(mPainted);
+  if (mPainted && mContext->IsDocumentLoading()) {
+    // It's possible that we get a DOMMetaAdded event after the page
+    // has already been painted, but before the document finishes loading.
+    // In such a case, we've already run SetInitialViewport() on
+    // "before-first-paint", and won't run it again on "load" (because
+    // mPainted=true). But that SetInitialViewport() call didn't know the
+    // "initial-scale" from this meta viewport tag. To ensure we respect
+    // the "initial-scale", call SetInitialViewport() again.
+    // Note: It's important that we only do this if mPainted=true. In the
+    // usual case, we get the DOMMetaAdded before the first paint, sometimes
+    // even before we have a frame tree, and calling SetInitialViewport()
+    // before we have a frame tree will skip some important steps (e.g.
+    // updating display port margins).
+    SetInitialViewport();
+  } else {
+    RefreshViewportSize(mPainted);
+  }
 }
 
 NS_IMETHODIMP