Bug 1729749 - Handle WM_NCLBUTTONDOWN on the maximize button only if the system title bar is hidden. r=handyman

Differential Revision: https://phabricator.services.mozilla.com/D125016

widget/windows/nsWindow.cpp

@@ -5928,7 +5928,7 @@ bool nsWindow::ProcessMessage(UINT msg, WPARAM& wParam, LPARAM& lParam,
 
       // WM_NCHITTEST handles HTMAXBUTTON so we need to handle clicks here.
       // WM_LBUTTONDOWNs will not be sent for this region.
-      if (wParam == HTMAXBUTTON) {
+      if (wParam == HTMAXBUTTON && mCustomNonClient) {
         DispatchMouseEvent(eMouseDown, wParam, lParamToClient(lParam), false,
                            MouseButton::ePrimary, MOUSE_INPUT_SOURCE());
         DispatchPendingEvents();