Steps to reproduce:
- Go to website (“Edit” mode) > Add a “Big Boxes” block.
- Switch to “Grid” mode > Select the last column n (while n > 1) and set
a background image on it.
- Try to change the background position > You cannot save or cancel the
changes since the option buttons (“Apply” & “Discard”) are invisible.
This is simply because the grid element was cloned on the background
container (`.o_overlay_background`) with its grid `z-index` style
(with `z-index` === n) which automatically hides the buttons.
The goal of this commit is to fix this behaviour by simply forcing the
`z-index` of the background element to 0.
task-3648466
closesodoo/odoo#150351
X-original-commit: 7dbb25dfeae25f27e10d4052db98c5f9d93699ff
Signed-off-by: Benjamin Vray (bvr) <bvr@odoo.com>