exo: Fix cursor size for multi-display ARC

components/exo/pointer.cc

 // Copyright 2015 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 "components/exo/pointer.h"
 
 #include "ash/public/cpp/shell_window_ids.h"
 #include "components/exo/pointer_delegate.h"
 #include "components/exo/pointer_stylus_delegate.h"
 #include "components/exo/surface.h"
@@ skipping 289 matching lines @@
   if (!target)
     return nullptr;
 
   return delegate_->CanAcceptPointerEventsForSurface(target) ? target : nullptr;
 }
 
 void Pointer::UpdateCursorScale() {
   if (!focus_)
     return;
 
+  display::Screen* screen = display::Screen::GetScreen();
+  WMHelper* helper = WMHelper::GetInstance();
+
   // Update cursor scale if the effective UI scale has changed.
   display::Display display =
-      display::Screen::GetScreen()->GetDisplayNearestWindow(
-          widget_->GetNativeWindow());
-  float ui_scale = WMHelper::GetInstance()
-                       ->GetDisplayInfo(display.id())
-                       .GetEffectiveUIScale();
-  if (WMHelper::GetInstance()->GetCursorSet() == ui::CURSOR_SET_LARGE)
+      screen->GetDisplayNearestWindow(widget_->GetNativeWindow());
+  float ui_scale = helper->GetDisplayInfo(display.id()).GetEffectiveUIScale();
+
+  // TODO(domlaskowski): Density in ARC is currently configured at build time,
+  // but it should be dynamic to support devices without an internal display.

[reveman, 2017/01/24 00:16:01]

nit: I don't think this TODO belongs here as the code that needs to be improved
is ARC and not this

[Dominik Laskowski, 2017/01/24 00:20:36]

Done.

+  if (display::Display::HasInternalDisplay()) {
+    float primary_device_scale_factor =
+        screen->GetPrimaryDisplay().device_scale_factor();
+
+    // The size of the cursor surface is the quotient of its physical size and
+    // the DSF of the primary display. The physical size is proportional to the
+    // DSF of the internal display. For external displays (and the internal
+    // display when secondary to a display with a different DSF), scale the
+    // cursor so its physical size matches with the single display case.
+    if (!display.IsInternal() ||
+        display.device_scale_factor() != primary_device_scale_factor) {
+      ui_scale *= primary_device_scale_factor /
+                  helper->GetDisplayInfo(display::Display::InternalDisplayId())
+                      .device_scale_factor();
+    }
+  }
+
+  if (helper->GetCursorSet() == ui::CURSOR_SET_LARGE)
     ui_scale *= kLargeCursorScale;
 
   if (ui_scale != cursor_scale_) {
     gfx::Transform transform;
     transform.Scale(ui_scale, ui_scale);
     widget_->GetNativeWindow()->SetTransform(transform);
     cursor_scale_ = ui_scale;
   }
 }
 
 }  // namespace exo