Windows shadows causing artefacts with the new Shell.BlurEffect

Affected version

Gnome 3.36.2, both Xorg and Wayland

Bug summary

When using the integrated blur effect Shell.BlurEffect with a background blur mode, moving windows around the 'themed' widget is currently causing artefacts.

Steps to reproduce

In the looking glass inspector (Alt+f2, lg):

1. Create a widget:

blurred_widget = new St.Widget({
  x: 0,
  y: 0,
  width: 100,
  height: 100,
});

2. Create a blur effect with mode 1 (blurs the background):

effect = new Shell.BlurEffect({
  brightness: 0.6,
  sigma: 30,
  mode: 1
});

3. Add the effect to the widget and place the widget:

blurred_widget.add_effect(effect);
Main.uiGroup.add_child(blurred_widget);

Exit the inspector (esc) and move windows around: when approaching from the widget, the windows shadows cause artefacts (even when not touching it entierely).

This is probably due to the effect not being repaint (?) because typing effect.queue_repaint() in the inspector resolves this problem temporarly and transforms that in a beautifully blurred widget.

I don't know if this is intended (after all, this effect is maybe not meant to be used this way), but it's sad because it is the only thing that prevents making an extension that adds "native" blur to the panel/dash (in dash to dock) or a lot of very interresting things.

Relevant logs, screenshots, screencasts etc.