[FIX] mail: tabs should appear on mobile

Since Chrome 74 or 75, tabs in Discuss suddenly disappeared.
Normally, these buttons are displayed at the bottom of the screen
when you open the Discuss app.

So we had to review this part and it seems more logical to override
the "height: 100%" property with "min-height: 0".
By doing this, the flex box element can grow from this size.

We also set flex-shrink to 1 to reduce the content with a scroll-bar
to let enough space to the other static elements (flex 0 0 auto).

Note that these rules are only applied on mobile.

opw-2030433

closes odoo/odoo#34947

Signed-off-by: Pierre Paridans <pparidans@users.noreply.github.com>

addons/mail/static/src/scss/discuss.scss

@@ -252,7 +252,8 @@
         }
 
         .o_mail_discuss_content {
-            flex: 1 0 0;
+            flex: 1 1 auto;
+            min-height: 0;
             margin-left: 10px;
             margin-right: 10px;
             background-color: transparent;