After commit ef46378, the position of the composer tools of the chatter
was below the text area instead of at the right of it. This is because
one div inside the flexbox was no longer diplayed as a flex or absolute.
By setting that disabled textbox to position absolute, the flexbox can
be displayed normaly again.
closes odoo/odoo#38183
Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>