Implemented smooth scrolling using xinput2 in X11.

chrome/browser/ui/views/frame/browser_root_view.cc

 // Copyright (c) 2012 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 "chrome/browser/ui/views/frame/browser_root_view.h"
 
+#include <cmath>
+
 #include "chrome/browser/autocomplete/autocomplete_classifier_factory.h"
 #include "chrome/browser/defaults.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/ui/browser_commands.h"
 #include "chrome/browser/ui/tabs/tab_strip_model.h"
 #include "chrome/browser/ui/views/frame/browser_frame.h"
 #include "chrome/browser/ui/views/frame/browser_view.h"
 #include "chrome/browser/ui/views/tabs/tab_strip.h"
 #include "chrome/browser/ui/views/touch_uma/touch_uma.h"
 #include "components/metrics/proto/omnibox_event.pb.h"
 #include "components/omnibox/browser/autocomplete_classifier.h"
 #include "components/omnibox/browser/autocomplete_match.h"
 #include "ui/base/dragdrop/drag_drop_types.h"
 #include "ui/base/dragdrop/os_exchange_data.h"
 #include "ui/base/hit_test.h"
 
 // static
 const char BrowserRootView::kViewClassName[] =
     "browser/ui/views/frame/BrowserRootView";
 
 BrowserRootView::BrowserRootView(BrowserView* browser_view,
                                  views::Widget* widget)
     : views::internal::RootView(widget),
       browser_view_(browser_view),
-      forwarding_to_tab_strip_(false) { }
+      forwarding_to_tab_strip_(false),
+      scroll_remainder_x_(0),
+      scroll_remainder_y_(0) {}
 
 bool BrowserRootView::GetDropFormats(
       int* formats,
       std::set<ui::Clipboard::FormatType>* format_types) {
   if (tabstrip() && tabstrip()->visible()) {
     *formats = ui::OSExchangeData::URL | ui::OSExchangeData::STRING;
     return true;
   }
   return false;
 }
@@ skipping 81 matching lines @@
 bool BrowserRootView::OnMouseWheel(const ui::MouseWheelEvent& event) {
   if (browser_defaults::kScrollEventChangesTab) {
     // Switch to the left/right tab if the wheel-scroll happens over the
     // tabstrip, or the empty space beside the tabstrip.
     views::View* hit_view = GetEventHandlerForPoint(event.location());
     int hittest =
         GetWidget()->non_client_view()->NonClientHitTest(event.location());
     if (tabstrip()->Contains(hit_view) ||
         hittest == HTCAPTION ||
         hittest == HTTOP) {
-      int scroll_offset = abs(event.y_offset()) > abs(event.x_offset()) ?
-          event.y_offset() : event.x_offset();
+      scroll_remainder_x_ += event.x_offset();
+      scroll_remainder_y_ += event.y_offset();
+
+      // Number of integer scroll events that have passed in each direction.
+      int whole_scroll_amount_x =
+          std::lround(static_cast<double>(scroll_remainder_x_) /
+                      ui::MouseWheelEvent::kWheelDelta);
+      int whole_scroll_amount_y =
+          std::lround(static_cast<double>(scroll_remainder_y_) /
+                      ui::MouseWheelEvent::kWheelDelta);
+
+      // Adjust the remainder such that any whole scrolls we have taken action
+      // for don't count towards the scroll remainder.
+      scroll_remainder_x_ -=
+          whole_scroll_amount_x * ui::MouseWheelEvent::kWheelDelta;
+      scroll_remainder_y_ -=
+          whole_scroll_amount_y * ui::MouseWheelEvent::kWheelDelta;
+
+      // Count a scroll in either axis - summing the axes works for this.
+      int whole_scroll_offset = whole_scroll_amount_x + whole_scroll_amount_y;
+
       Browser* browser = browser_view_->browser();
       TabStripModel* model = browser->tab_strip_model();
       // Switch to the next tab only if not at the end of the tab-strip.
-      if (scroll_offset < 0 && model->active_index() + 1 < model->count()) {
+      if (whole_scroll_offset < 0 &&
+          model->active_index() + 1 < model->count()) {
         chrome::SelectNextTab(browser);
         return true;
       }
 
       // Switch to the previous tab only if not at the beginning of the
       // tab-strip.
-      if (scroll_offset > 0 && model->active_index() > 0) {
+      if (whole_scroll_offset > 0 && model->active_index() > 0) {
         chrome::SelectPreviousTab(browser);
         return true;
       }
     }
   }
   return RootView::OnMouseWheel(event);
 }
 
+void BrowserRootView::OnMouseExited(const ui::MouseEvent& event) {

[msw, 2016/02/02 19:00:57]

This should probably be calling views::internal::RootView::OnMouseExited.
(sorry I missed this during review, it seems to have caused crbug.com/583380)

+  // Reset such that the tab switch always occurs halfway through a smooth
+  // scroll.
+  scroll_remainder_x_ = 0;
+  scroll_remainder_y_ = 0;
+}
+
 void BrowserRootView::OnEventProcessingStarted(ui::Event* event) {
   if (event->IsGestureEvent()) {
     ui::GestureEvent* gesture_event = event->AsGestureEvent();
     if (gesture_event->type() == ui::ET_GESTURE_TAP &&
         gesture_event->location().y() <= 0 &&
         gesture_event->location().x() <= browser_view_->GetBounds().width()) {
       TouchUMA::RecordGestureAction(TouchUMA::GESTURE_ROOTVIEWTOP_TAP);
     }
   }
 
@@ skipping 40 matching lines @@
       browser_view_->browser()->profile())->Classify(
           text, false, false, metrics::OmniboxEventProto::INVALID_SPEC, &match,
           nullptr);
   if (!match.destination_url.is_valid())
     return false;
 
   if (url)
     *url = match.destination_url;
   return true;
 }