[iOS] Show toolbars after scrolling to the bottom of the page.

This CL updates the behavior of fullscreen such that the toolbars will
be re-shown when the user starts a scroll past the bottom of a page
when the toolbars are completely collapsed.

Bug: 841094
Cq-Include-Trybots: luci.chromium.try:ios-simulator-cronet;luci.chromium.try:ios-simulator-full-configs
Change-Id: I9e2cb3a59e931f83bb43e8a68df48a80bb6d0c3b
Reviewed-on: https://chromium-review.googlesource.com/1200563
Commit-Queue: Kurt Horimoto <kkhorimoto@chromium.org>
Reviewed-by: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589357}

ios/chrome/browser/ui/browser_view_controller.mm

@@ -4150,6 +4150,8 @@ applicationCommandEndpoint:(id<ApplicationCommands>)applicationCommandEndpoint {
   self.footerFullscreenProgress = progress;
 
   if (IsUIRefreshPhase1Enabled()) {
+    // TODO(crbug.com/880656): Update implementation to make the bottom toolbar
+    // animatable.
     self.secondaryToolbarHeightConstraint.constant =
         [self secondaryToolbarHeightWithInset] * progress;
 

ios/chrome/browser/ui/fullscreen/fullscreen_mediator.mm

@@ -120,6 +120,13 @@ void FullscreenMediator::FullscreenModelScrollEventStarted(
     FullscreenModel* model) {
   DCHECK_EQ(model_, model);
   StopAnimating(true /* update_model */);
+  // Show the toolbars if the user begins a scroll past the bottom edge of the
+  // screen and the toolbars have been fully collapsed.
+  if (model_->is_scrolled_to_bottom() &&
+      AreCGFloatsEqual(model_->progress(), 0.0) &&
+      model_->can_collapse_toolbar()) {
+    AnimateModelReset();
+  }
 }
 
 void FullscreenMediator::FullscreenModelScrollEventEnded(

ios/chrome/browser/ui/fullscreen/fullscreen_model.h

@@ -47,6 +47,17 @@ class FullscreenModel : public ChromeBroadcastObserverInterface {
     return expanded_toolbar_height_ - collapsed_toolbar_height_;
   }
 
+  // Returns whether the page content is tall enough for the toolbar to be
+  // scrolled to an entirely collapsed position.
+  bool can_collapse_toolbar() const {
+    return content_height_ > scroll_view_height_ + toolbar_height_delta();
+  }
+
+  // Whether the view is scrolled all the way to the bottom.
+  bool is_scrolled_to_bottom() const {
+    return y_content_offset_ + scroll_view_height_ >= content_height_;
+  }
+
   // Increments and decrements |disabled_counter_| for features that require the
   // toolbar be completely visible.
   void IncrementDisabledCounter();

ios/chrome/browser/ui/fullscreen/fullscreen_model.mm

@@ -224,17 +224,14 @@ FullscreenModel::ScrollAction FullscreenModel::ActionForScrollFromOffset(
   // Ignore if:
   // - explicitly requested via IgnoreRemainderOfCurrentScroll(),
   // - the scroll is a bounce-up animation at the top,
-  // - the scroll is a bounce-down animation at the bottom,
+  // - the scroll is attempting to scroll past the bottom of the page,
   // - the scroll is attempting to scroll content up when it already fits.
   bool scrolling_content_down = y_content_offset_ - from_offset < 0.0;
   bool scrolling_past_top = y_content_offset_ <= -top_inset_;
-  bool scrolling_past_bottom =
-      y_content_offset_ + scroll_view_height_ >= content_height_;
   bool content_fits = content_height_ <= scroll_view_height_ - top_inset_;
   if (ignoring_current_scroll_ ||
       (scrolling_past_top && !scrolling_content_down) ||
-      (scrolling_past_bottom && scrolling_content_down) ||
-      (content_fits && !scrolling_content_down)) {
+      is_scrolled_to_bottom() || (content_fits && !scrolling_content_down)) {
     return ScrollAction::kIgnore;
   }