Decorations: Adjust windows rect to have resize area outside frame

We should give the os enough space outside the frame to handle the
resize area itself.
This avoids issues with scrollbars at the window edge etc.
Fixes #266

windows/src/main/cpp/Decorations.cpp

@@ -49,6 +49,10 @@ static bool IsLeftMousePressed(WindowWrapper *wrapper) {
     }
 }
 
+static inline int GetFrameSize() {
+    return GetSystemMetrics(SM_CXFRAME) + GetSystemMetrics(SM_CXPADDEDBORDER);
+}
+
 static LRESULT HandleHitTest(WindowWrapper *wrapper, int x, int y) {
     if (wrapper->popup_menu) return HTCLIENT;
 
@@ -67,7 +71,7 @@ static LRESULT HandleHitTest(WindowWrapper *wrapper, int x, int y) {
          * The horizontal frame should be the same size as the vertical frame,
          * since the NONCLIENTMETRICS structure does not distinguish between them
          */
-        int frame_size = GetSystemMetrics(SM_CXFRAME) + GetSystemMetrics(SM_CXPADDEDBORDER);
+        int frame_size = GetFrameSize();
         // The diagonal size handles are wider than the frame
         int diagonal_width = frame_size * 2 + GetSystemMetrics(SM_CXBORDER);
 
@@ -226,6 +230,12 @@ static void HandleNCCalcSize(WindowWrapper *wrapper, WPARAM wparam, LPARAM lpara
          * before WM_NCCALCSIZE modified it. This will make the client size the
          * same as the non-client size.
          */
+        if (wrapper->resizable) {
+            int frame_size = GetFrameSize();
+            nonclient.left += frame_size;
+            nonclient.right -= frame_size;
+            nonclient.bottom -= frame_size;
+        }
         *params.rect = nonclient;
     }
 }