WIP: Tab-Drag-Slippage Various experiments

ui/views/win/hwnd_message_handler.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 "ui/views/win/hwnd_message_handler.h"
 
 #include <dwmapi.h>
 #include <oleacc.h>
 #include <shellapi.h>
 
@@ skipping 677 matching lines @@
 
 bool HWNDMessageHandler::RunMoveLoop(const gfx::Vector2d& drag_offset,
                                      bool hide_on_escape) {
   ReleaseCapture();
   MoveLoopMouseWatcher watcher(this, hide_on_escape);
   // In Aura, we handle touch events asynchronously. So we need to allow nested
   // tasks while in windows move loop.
   base::MessageLoop::ScopedNestableTaskAllower allow_nested(
       base::MessageLoop::current());
 
-  SendMessage(hwnd(), WM_SYSCOMMAND, SC_MOVE | 0x0002, GetMessagePos());
+  // SC_MOVE ignores the lParam cursor argument, so first ensure the point
+  // |drag_offset| into the window lies under the cursor. Note that the offset
+  // is in window coordinates, the cursor in screen coordinates, and the call to
+  // SetWindowPos must be client area coordinates.
+  DWORD message_pos = GetMessagePos();
+  POINTS cursor_pos = MAKEPOINTS(message_pos);
+  POINT anchor_pos = { drag_offset.x(), drag_offset.y() };
+  //MapWindowPoints(hwnd(), HWND_DESKTOP, &anchor_pos, 1);
+  ClientToScreen(hwnd(), &anchor_pos);
+  RECT client_rect;
+  GetClientRect(hwnd(), &client_rect);
+  SetWindowPos(hwnd(), nullptr,
+               client_rect.left + cursor_pos.x - anchor_pos.x,
+               client_rect.top + cursor_pos.y - anchor_pos.y,
+               0, 0, SWP_NOSIZE);
+
+  SendMessage(hwnd(), WM_SYSCOMMAND, SC_MOVE | 0x0002, 0);
   // Windows doesn't appear to offer a way to determine whether the user
   // canceled the move or not. We assume if the user released the mouse it was
   // successful.
   return watcher.got_mouse_up();
 }
 
 void HWNDMessageHandler::EndMoveLoop() {
   SendMessage(hwnd(), WM_CANCELMODE, 0, 0);
 }
 
@@ skipping 2030 matching lines @@
     SetWindowPos(hwnd(), NULL, 0, 0, 0, 0, flags | SWP_HIDEWINDOW);
     SetWindowPos(hwnd(), NULL, 0, 0, 0, 0, flags | SWP_SHOWWINDOW);
   }
   // WM_DWMCOMPOSITIONCHANGED is only sent to top level windows, however we want
   // to notify our children too, since we can have MDI child windows who need to
   // update their appearance.
   EnumChildWindows(hwnd(), &SendDwmCompositionChanged, NULL);
 }
 
 }  // namespace views