[FIX] Website: fix mega menu not visible on mobile device

Steps to reproduce the bug:

- Open the website editor.
- Create a mega menu.
- Save changes.
- Open the website in mobile.
- Scroll down to the bottom of the page.
- Open the hamburger menu.
- Click the mega menu link.
- The mega menu doesn't open.

The bug was introduced by commit [1], when the mega menu in the mobile
navbar was changed to "position: fixed".

The issue happens because when the page is scrolled, the header gets a
"transform: translate" applied to it. This creates a new coordinate
system, which breaks the reference for the mega menu’s fixed
positioning. As a result, the menu has no reliable reference to
calculate its height.

This commit adds a CSS rule setting the height to 100vh as a fix. It
ensures the mega menu always takes the full viewport height, regardless
of the transformed parent.

[1]: odoo@dc1a155

opw-4747373

closes odoo#210864

X-original-commit: 579160b
Signed-off-by: Serge Bayet (seba) <[email protected]>
Signed-off-by: Benjamin Vray (bvr) <[email protected]>

Author: bvr-odoo

addons/website/static/src/scss/website.scss

@@ -1782,6 +1782,7 @@ header {
         visibility: hidden;
         margin-top: $o-mega-menu-nav-height !important;
         border: none;
+        height: 100vh;
         max-height: 100vh;
         background-color: o-color('menu-custom') or o-color('menu');
         overflow-x: hidden;