[BUG] :stacking "bg" doesn't work as expected in BSPWM

[andornaut]

Checklist before submitting an issue

• I have searched through the existing closed and open issues for eww and made sure this is not a duplicate
• I have specifically verified that this bug is not a common user error
• I am providing as much relevant information as I am able to in this bug report (Minimal config to reproduce the issue for example, if applicable)

Description of the bug

Floating or maximized are rendered above an eww bar configured with :stacking "bg", whereas I'd expect these to be rendered above the eww bar.

Here's a screenshot showing a calculator app in a floating window being rendered behind the eww bar:

This also occurs with "maximized" windows: the maximized window is rendered below instead of above the eww bar.

Reproducing the issue

1. Configure a bar with :stacking "bg"

(defwindow bar
  :exclusive true
  :focusable "none"
  :geometry (geometry
    :anchor "top left"
    :width "48px"
    :height "100%"
  )
  :monitor 0
  :reserve (struts :distance "49px" :side "left")
  :stacking "bg"

2. Intersect a floating window with the eww bar

Expected behaviour

Expected behaviour: The floating window should be rendered above the eww bar

Actual behaviour: the floating window is rendered below the eww bar

Additional context

No response

Platform and environment

bspwm 0.9.10
eww 0.6.0 fddb4a0
linux kernel 6.14.0-29
distribution: Ubuntu 24.04.3 LTS
xorg 1:7.7+23ubuntu3

My eww configuration is available in my public dotfiles