Controls offsets computed if either top or bottom are showing

cc/input/browser_controls_offset_manager.cc

Index: cc/input/browser_controls_offset_manager.cc
diff --git a/cc/input/browser_controls_offset_manager.cc b/cc/input/browser_controls_offset_manager.cc
index 841aad39bb628d02192e735106c7a863c87c5424..5a970627bc302658ec7bb9a2a115c4f60916f48f 100644
--- a/cc/input/browser_controls_offset_manager.cc
+++ b/cc/input/browser_controls_offset_manager.cc
@@ -43,7 +43,8 @@ BrowserControlsOffsetManager::BrowserControlsOffsetManager(
       animation_direction_(NO_ANIMATION),
       permitted_state_(BOTH),
       accumulated_scroll_delta_(0.f),
-      baseline_content_offset_(0.f),
+      baseline_top_content_offset_(0.f),
+      baseline_bottom_content_offset_(0.f),
       controls_show_threshold_(controls_hide_threshold),
       controls_hide_threshold_(controls_show_threshold),
       pinch_gesture_active_(false) {
@@ -60,12 +61,6 @@ float BrowserControlsOffsetManager::ContentTopOffset() const {
   return TopControlsShownRatio() * TopControlsHeight();
 }
 
-float BrowserControlsOffsetManager::ContentOffsetInternal() const {
-  if (!TopControlsHeight())
-    return BottomControlsShownRatio() * BottomControlsHeight();
-  return ContentTopOffset();
-}
-
 float BrowserControlsOffsetManager::TopControlsShownRatio() const {
   return client_->CurrentBrowserControlsShownRatio();
 }
@@ -144,9 +139,11 @@ gfx::Vector2dF BrowserControlsOffsetManager::ScrollBy(
 
   accumulated_scroll_delta_ += pending_delta.y();
 
-  float old_offset = ContentOffsetInternal();
+  float old_top_offset = ContentTopOffset();
+  float baseline_content_offset = TopControlsHeight()

[aelias_OOO_until_Jul13, 2016/11/01 21:15:46]

I think this should be structured more symmetrically.  How about structuring
this in a way that you check BottomControlsHeight() and potentially act twice? 
Even if we're not planning to support the "both bottom and top" use case, I
think the code would be easier to understand that way (one less arbitrary
assumption baked in).

+      ? baseline_top_content_offset_ : baseline_bottom_content_offset_;
   client_->SetCurrentBrowserControlsShownRatio(
-      (baseline_content_offset_ - accumulated_scroll_delta_) / controls_height);
+      (baseline_content_offset - accumulated_scroll_delta_) / controls_height);
 
   // If the controls are fully visible, treat the current position as the
   // new baseline even if the gesture didn't end.
@@ -155,7 +152,12 @@ gfx::Vector2dF BrowserControlsOffsetManager::ScrollBy(
 
   ResetAnimations();
 
-  gfx::Vector2dF applied_delta(0.f, old_offset - ContentOffsetInternal());
+  // applied_delta will negate any scroll on the content if the top browser
+  // controls are showing in favor of hiding the controls and resizing the
+  // content. If the top controls have no height, the content should scroll
+  // immediately.
+  gfx::Vector2dF applied_delta(0.f,
+      TopControlsHeight() ? old_top_offset - ContentTopOffset() : 0.0f);
   return pending_delta - applied_delta;
 }
 
@@ -189,7 +191,7 @@ gfx::Vector2dF BrowserControlsOffsetManager::Animate(
   if (!has_animation() || !client_->HaveRootScrollLayer())
     return gfx::Vector2dF();
 
-  float old_offset = ContentOffsetInternal();
+  float old_offset = ContentTopOffset();
   float new_ratio = gfx::Tween::ClampedFloatValueBetween(
       monotonic_time, animation_start_time_, animation_start_value_,
       animation_stop_time_, animation_stop_value_);
@@ -198,7 +200,9 @@ gfx::Vector2dF BrowserControlsOffsetManager::Animate(
   if (IsAnimationComplete(new_ratio))
     ResetAnimations();
 
-  gfx::Vector2dF scroll_delta(0.f, ContentOffsetInternal() - old_offset);
+  // Don't move the content if the top controls aren't used.
+  gfx::Vector2dF scroll_delta(0.f,
+      TopControlsHeight() ? (ContentTopOffset() - old_offset) : 0.0f);
   return scroll_delta;
 }
 
@@ -265,7 +269,8 @@ bool BrowserControlsOffsetManager::IsAnimationComplete(float new_ratio) {
 
 void BrowserControlsOffsetManager::ResetBaseline() {
   accumulated_scroll_delta_ = 0.f;
-  baseline_content_offset_ = ContentOffsetInternal();
+  baseline_top_content_offset_ = ContentTopOffset();
+  baseline_bottom_content_offset_ = ContentBottomOffset();
 }
 
 }  // namespace cc