WIP: Tab-Drag-Slippage Various experiments

ui/views/win/hwnd_message_handler.cc

Index: ui/views/win/hwnd_message_handler.cc
diff --git a/ui/views/win/hwnd_message_handler.cc b/ui/views/win/hwnd_message_handler.cc
index bd2072b47ee7d0b71d538d9f065151da85262315..23f09dcf08871f9814b8d2184d100e5de0568f3e 100644
--- a/ui/views/win/hwnd_message_handler.cc
+++ b/ui/views/win/hwnd_message_handler.cc
@@ -695,7 +695,23 @@ bool HWNDMessageHandler::RunMoveLoop(const gfx::Vector2d& drag_offset,
   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.