Controls offsets computed if either top or bottom are showing

cc/input/browser_controls_offset_manager.cc

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "cc/input/browser_controls_offset_manager.h"
 
 #include <stdint.h>
 
 #include <algorithm>
 
@@ skipping 25 matching lines @@
 BrowserControlsOffsetManager::BrowserControlsOffsetManager(
     BrowserControlsOffsetManagerClient* client,
     float controls_show_threshold,
     float controls_hide_threshold)
     : client_(client),
       animation_start_value_(0.f),
       animation_stop_value_(0.f),
       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) {
   CHECK(client_);
 }
 
 BrowserControlsOffsetManager::~BrowserControlsOffsetManager() {}
 
 float BrowserControlsOffsetManager::ControlsTopOffset() const {
   return ContentTopOffset() - TopControlsHeight();
 }
 
 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();
 }
 
 float BrowserControlsOffsetManager::TopControlsHeight() const {
   return client_->TopControlsHeight();
 }
 
 float BrowserControlsOffsetManager::BottomControlsHeight() const {
   return client_->BottomControlsHeight();
@@ skipping 58 matching lines @@
   if (pinch_gesture_active_)
     return pending_delta;
 
   if (permitted_state_ == SHOWN && pending_delta.y() > 0)
     return pending_delta;
   else if (permitted_state_ == HIDDEN && pending_delta.y() < 0)
     return pending_delta;
 
   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.
   if (TopControlsShownRatio() == 1.f)
     ResetBaseline();
 
   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;
 }
 
 void BrowserControlsOffsetManager::ScrollEnd() {
   if (pinch_gesture_active_)
     return;
 
   StartAnimationIfNecessary();
 }
 
@@ skipping 13 matching lines @@
 
 void BrowserControlsOffsetManager::MainThreadHasStoppedFlinging() {
   StartAnimationIfNecessary();
 }
 
 gfx::Vector2dF BrowserControlsOffsetManager::Animate(
     base::TimeTicks monotonic_time) {
   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_);
   client_->SetCurrentBrowserControlsShownRatio(new_ratio);
 
   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;
 }
 
 void BrowserControlsOffsetManager::ResetAnimations() {
   animation_start_time_ = base::TimeTicks();
   animation_start_value_ = 0.f;
   animation_stop_time_ = base::TimeTicks();
   animation_stop_value_ = 0.f;
 
   animation_direction_ = NO_ANIMATION;
@@ skipping 46 matching lines @@
   }
 }
 
 bool BrowserControlsOffsetManager::IsAnimationComplete(float new_ratio) {
   return (animation_direction_ == SHOWING_CONTROLS && new_ratio >= 1.f) ||
          (animation_direction_ == HIDING_CONTROLS && new_ratio <= 0.f);
 }
 
 void BrowserControlsOffsetManager::ResetBaseline() {
   accumulated_scroll_delta_ = 0.f;
-  baseline_content_offset_ = ContentOffsetInternal();
+  baseline_top_content_offset_ = ContentTopOffset();
+  baseline_bottom_content_offset_ = ContentBottomOffset();
 }
 
 }  // namespace cc