exo: Fix cursor size for multi-display ARC

components/exo/pointer.cc

Index: components/exo/pointer.cc
diff --git a/components/exo/pointer.cc b/components/exo/pointer.cc
index b10d0186d798e64a6b47f7ca69e1dd9fe5b95bac..bd267e04bc6483bdfa5c4ad0d23add376fc1ff30 100644
--- a/components/exo/pointer.cc
+++ b/components/exo/pointer.cc
@@ -307,14 +307,32 @@ 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();
+
+  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() ||

[oshima, 2017/01/24 20:48:19]

Isn't this && ? 

[Dominik Laskowski, 2017/01/24 22:14:34]

No, we want to avoid scaling if:

1) The window is on the internal display.
2) The external display is primary.
3) The DSFs are equal.

For example, when using native resolution on Samus, the correct cursor size is
50x50 pixels because the DSF is 1 (see comment below). If a low-DPI external
display is primary, the cursor should not be scaled to 25x25. If the external
display is high-DPI, then the division below cancels out, so the cursor is not
scaled either.

[oshima, 2017/01/25 21:16:01]

The size of the cursor should be same (50x50 pixels?) regardless of DSF for
HighDPI internal display.
(That is 50x50 DIP when internal display is 1x, but 25x25 DIP when internal
display is 2x)
Hmm, I'm confused. Can you use DIP or pixels always? I thought on low DIP the
cursor should
be 25x25 pixels.
Let's have a quick offline chat. I probably don't know exactly how cursor works
on Android side,
but I want to make sure I fully understand this.

+        display.device_scale_factor() != primary_device_scale_factor) {

[oshima, 2017/01/24 20:48:19]

My understanding was that if the android scale factor (which is determined by
the internal display's native scale factor) is not consistent with external
display's one we need to scale it back.
Am I wrong?

[Dominik Laskowski, 2017/01/24 22:14:34]

It's a bit more complicated because:

1) HWC divides the cursor size (50x50) by the primary DSF, so that (2) is
satisfied for the single high-DPI display case.

2) For high-DPI internal displays, the cursor size at native resolution is
specified in pixels rather than DIPs. For example, the size is 50x50 pixels for
native resolution, and 25x25 DIPs for all other resolutions.

+      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_) {