private int getResizeHandleHeight() {
			int dpi = User32Ex.INSTANCE.GetDpiForWindow( hwnd );

			// there isn't a SM_CYPADDEDBORDER for the Y axis
			return User32Ex.INSTANCE.GetSystemMetricsForDpi( SM_CXPADDEDBORDER, dpi )
				 + User32Ex.INSTANCE.GetSystemMetricsForDpi( SM_CYSIZEFRAME, dpi );
		}
Returns the height of the little space at the top of the window used to
resize the window.

See also NonClientIslandWindow::_GetResizeHandleHeight() here:
https://github.com/microsoft/terminal/blob/main/src/cascadia/WindowsTerminal/NonClientIslandWindow.cpp
flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java