internal/devicescale: Add comment about #1573

Updates #1573

internal/devicescale/devicescale.go

@@ -37,5 +37,11 @@ func GetAt(x, y int) float64 {
 	}
 	s := impl(x, y)
 	cache[pos{x, y}] = s
+
+	// TODO: Provide a way to invalidate the cache, or move the cache.
+	// The device scale can vary even for the same monitor.
+	// The only known case is when the application works on macOS, with OpenGL, with a wider screen mode,
+	// and in the fullscreen mode (#1573).
+
 	return s
 }