exo: Fix dragging edge cases

components/exo/display.h

Index: components/exo/display.h
diff --git a/components/exo/display.h b/components/exo/display.h
index d4f3b3d08065b522f902612fe7fabe6c23a806de..6b653592f95f344907deb02c4fd61a661c826c21 100644
--- a/components/exo/display.h
+++ b/components/exo/display.h
@@ -12,6 +12,7 @@
 
 #include "base/macros.h"
 #include "base/memory/shared_memory_handle.h"
+#include "ui/gfx/geometry/point.h"
 
 #if defined(USE_OZONE)
 #include "base/files/scoped_file.h"
@@ -20,10 +21,6 @@
 #include "ui/gfx/native_pixmap_handle.h"
 #endif
 
-namespace gfx {
-class Point;
-}
-
 namespace exo {
 class NotificationSurface;
 class NotificationSurfaceManager;
@@ -39,12 +36,45 @@ class Buffer;
 // The core display class. This class provides functions for creating surfaces
 // and is in charge of combining the contents of multiple surfaces into one
 // displayable output.
+//
+// This class is also responsible for conversion between server-side screen
+// coordinates and client-side virtual coordinates. This mapping enables
+// support for multiple displays when the client is limited to a single
+// display. In that case, the client uses a virtual display computed as
+// the bounding box of the physical displays, and the server translates
+// positions based on the display layout. For example, P is the client's
+// origin in virtual coordinates, and Q is the server's origin in screen
+// coordinates.
+//
+//  P        Q
+//  +-----+ /
+//  |     |/
+//  |  2  +-----------+
+//  |     |           |
+//  +-----+     1     |
+//        |           |
+//        +-----------+
+//
 class Display {
  public:
   Display();
   explicit Display(NotificationSurfaceManager* notification_surface_manager);
   ~Display();
 
+  void set_virtual_origin(const gfx::Point& virtual_origin) {
+    virtual_origin_ = virtual_origin;
+  }
+
+  template <typename Point>

[reveman, 2016/10/16 23:16:02]

why the template argument? what are the different point types that are passed to
this function?

[Dominik Laskowski, 2016/10/18 21:22:56]

gfx::Point and gfx::Rect. Would overloads be preferable?

+  void ConvertFromVirtualToScreen(Point* point) const {
+    *point += virtual_origin_.OffsetFromOrigin();
+  }
+
+  template <typename Point>
+  void ConvertFromScreenToVirtual(Point* point) const {
+    *point -= virtual_origin_.OffsetFromOrigin();
+  }
+
   // Creates a new surface.
   std::unique_ptr<Surface> CreateSurface();
 
@@ -88,6 +118,9 @@ class Display {
       const std::string& notification_id);
 
  private:
+  // Origin of the virtual screen relative to the primary display.
+  gfx::Point virtual_origin_;
+
   NotificationSurfaceManager* const notification_surface_manager_;
 
   DISALLOW_COPY_AND_ASSIGN(Display);