Fixes UI jank caused by footer visibility change.

By adding a parent container to |footer_|, we will reserve the same
amount of layout space whether or not that view is visible. This is
because the parent container uses a FillLayout which does not factor
visibility in when measuring.

Bug: b:113871610
Change-Id: I4810368f5de3af4c3b8152e080acf7c2249304fc
Reviewed-on: https://chromium-review.googlesource.com/1219899Reviewed-by: Xiaohui Chen <xiaohuic@chromium.org>
Commit-Queue: David Black <dmblack@google.com>
Cr-Commit-Position: refs/heads/master@{#590467}

ash/assistant/ui/main_stage/assistant_footer_view.cc

@@ -82,8 +82,6 @@ void AssistantFooterView::InitLayout() {
   const bool setup_completed =
       Shell::Get()->voice_interaction_controller()->setup_completed();
 
-  LOG(ERROR) << "eyor: setup_completed: " << setup_completed;
-
   // Suggestion container.
   suggestion_container_ = new SuggestionContainerView(assistant_controller_);
   suggestion_container_->set_can_process_events_within_subtree(setup_completed);
@@ -115,8 +113,6 @@ void AssistantFooterView::OnVoiceInteractionSetupCompleted(bool completed) {
   using assistant::util::CreateOpacityElement;
   using assistant::util::StartLayerAnimationSequence;
 
-  LOG(ERROR) << "eyor: completed: " << completed;
-
   // When the consent state changes, we need to hide/show the appropriate views.
   views::View* hide_view =
       completed ? static_cast<views::View*>(opt_in_view_)
@@ -134,6 +130,7 @@ void AssistantFooterView::OnVoiceInteractionSetupCompleted(bool completed) {
   StartLayerAnimationSequence(hide_view->layer()->GetAnimator(),
                               CreateLayerAnimationSequence(CreateOpacityElement(
                                   0.f, kAnimationFadeOutDuration)),
+                              // Observe the animation.
                               animation_observer_.get());
 
   // Show the view for the next consent state by fading to 100% opacity with

ash/assistant/ui/main_stage/assistant_main_stage.cc

@@ -254,6 +254,13 @@ void AssistantMainStage::InitContentLayoutContainer() {
   layout_manager->SetFlexForView(ui_element_container_, 1);
 
   // Footer.
+  // Note that the |footer_| is placed within its own view container so that as
+  // its visibility changes, its parent container will still reserve the same
+  // layout space. This prevents jank that would otherwise occur due to
+  // |ui_element_container_| claiming that empty space.
+  views::View* footer_container = new views::View();
+  footer_container->SetLayoutManager(std::make_unique<views::FillLayout>());
+
   footer_ = new AssistantFooterView(assistant_controller_);
   footer_->AddObserver(this);
 
@@ -261,7 +268,8 @@ void AssistantMainStage::InitContentLayoutContainer() {
   footer_->SetPaintToLayer();
   footer_->layer()->SetFillsBoundsOpaquely(false);
 
-  content_layout_container_->AddChildView(footer_);
+  footer_container->AddChildView(footer_);
+  content_layout_container_->AddChildView(footer_container);
 
   AddChildView(content_layout_container_);
 }