[UI] [Win] Fix scaling constant in mouse move

To convert from screen coordinates to the normalized absolute
coordinates dimensions used by SendInput, the scaling factor should be
(65536.0 / screen_dimension).

Previously, we used (65535.0 / (screen_dimension - 1)). Although the
difference between these two factors is small, the discrepancy becomes
more prominent when it's multiplied out. This discrepancy may lead to
the computed absolute coordinates being off by a pixel or two depending
on the resolution of the screen in question.

Change-Id: Id305abc6ddf904ffbbb07dc740c655ac1be3cf83
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2278347Reviewed-by: David Bienvenu <davidbienvenu@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Reviewed-by: Greg Thompson <grt@chromium.org>
Commit-Queue: Chege Gitau <dagitau@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#786075}

ui/base/win/event_creation_utils.cc

@@ -18,18 +18,24 @@ bool SendMouseEvent(const gfx::Point& point, int flags) {
   INPUT input = {INPUT_MOUSE};
   // Get the max screen coordinate for use in computing the normalized absolute
   // coordinates required by SendInput.
-  const int max_x = ::GetSystemMetrics(SM_CXSCREEN) - 1;
-  const int max_y = ::GetSystemMetrics(SM_CYSCREEN) - 1;
-  int screen_x = base::ClampToRange(point.x(), 0, max_x);
-  int screen_y = base::ClampToRange(point.y(), 0, max_y);
+  const int screen_width = ::GetSystemMetrics(SM_CXSCREEN);
+  const int screen_height = ::GetSystemMetrics(SM_CYSCREEN);
+  int screen_x = base::ClampToRange(point.x(), 0, screen_width - 1);
+  int screen_y = base::ClampToRange(point.y(), 0, screen_height - 1);
+
+  // In normalized absolute coordinates, (0, 0) maps onto the upper-left corner
+  // of the display surface, while (65535, 65535) maps onto the lower-right
+  // corner.
+  // https://docs.microsoft.com/en-us/windows/win32/api/winuser/nf-winuser-mouse_event#remarks
+  static constexpr double kNormalizedScreenSize = 65536.0;
 
   // Form the input data containing the normalized absolute coordinates. As of
   // Windows 10 Fall Creators Update, moving to an absolute position of zero
   // does not work. It seems that moving to 1,1 does, though.
-  input.mi.dx =
-      static_cast<LONG>(std::max(1.0, std::ceil(screen_x * (65535.0 / max_x))));
-  input.mi.dy =
-      static_cast<LONG>(std::max(1.0, std::ceil(screen_y * (65535.0 / max_y))));
+  input.mi.dx = static_cast<LONG>(std::max(
+      1.0, std::ceil(screen_x * (kNormalizedScreenSize / screen_width))));
+  input.mi.dy = static_cast<LONG>(std::max(
+      1.0, std::ceil(screen_y * (kNormalizedScreenSize / screen_height))));
   input.mi.dwFlags = flags;
   return ::SendInput(1, &input, sizeof(input)) == 1;
 }