Long story short: that menu loading effect was introduced with version
12.0 for a technical reason: allow menu items to collapse into a
dropdown when there are too many. For that, during loading their height
is forced to 0 and they are hidden so that they do not make the header
grow when going on multiple lines.

Since 13.0, the lazy loading made this effect to be too delayed... but
fixing that would make another flickering appear due to the logo image
loading. This thus cannot be fixed at the moment.

However, since 14.0 there are several menu templates and some make the
effect look really ugly. Like the vertical header which has a tiny
empty header which then grow with the menu items on loading. To mitigate
the effect, this commit replaces the "height: 0" trick by forcing the
menu items on one line during loading. They however still stay hidden
during this time to avoid the logo menu flickering, for now.

Part of https://github.com/odoo/odoo/pull/59136



closes odoo/odoo#59136

Signed-off-by: Quentin Smetz (qsm) <qsm@odoo.com>