exo: Fix dragging edge cases

components/exo/display.h

 // 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.
 
 #ifndef COMPONENTS_EXO_DISPLAY_H_
 #define COMPONENTS_EXO_DISPLAY_H_
 
 #include <stddef.h>
 
 #include <memory>
 #include <string>
 
 #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"
 #include "ui/gfx/buffer_types.h"
 #include "ui/gfx/geometry/size.h"
 #include "ui/gfx/native_pixmap_handle.h"
 #endif
 
-namespace gfx {
-class Point;
-}
-
 namespace exo {
 class NotificationSurface;
 class NotificationSurfaceManager;
 class SharedMemory;
 class ShellSurface;
 class SubSurface;
 class Surface;
 
 #if defined(USE_OZONE)
 class Buffer;
 #endif
 
 // 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();
 
   // Creates a shared memory segment from |handle| of |size| with the
   // given |id|. This function takes ownership of |handle|.
   std::unique_ptr<SharedMemory> CreateSharedMemory(
       const base::SharedMemoryHandle& handle,
       size_t size);
 
 #if defined(USE_OZONE)
@@ skipping 23 matching lines @@
   // a child of |parent|.
   std::unique_ptr<SubSurface> CreateSubSurface(Surface* surface,
                                                Surface* parent);
 
   // Creates a notification surface for a surface and notification id.
   std::unique_ptr<NotificationSurface> CreateNotificationSurface(
       Surface* surface,
       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);
 };
 
 }  // namespace exo
 
 #endif  // COMPONENTS_EXO_DISPLAY_H_