LoginApp not shown if kibana is deployed with a base path starting with /app

[g-w]

Kibana version:
7.7.0
Elasticsearch version:
7.7.0
Server OS version:
Debian 10
Browser version:
Firefox / Google Chrome
Browser OS version:
Fedora
Original install method (e.g. download page, yum, from source, etc.):
Docker image
Describe the bug:
If kibana is configured to use a server.basePath starting with /app (ie. /apps/kibana) the LoginApp provided by the security plugin is not shown.
Steps to reproduce:

1. Install elastic search with a security configuration
2. Configure kibana with server.basePath: /apps/kibana
3. Visit /apps/kibana

Expected behavior:
The login form shows.

Any additional context:
This issue is related to https://github.com/elastic/kibana/blob/7.7/src/core/public/application/ui/app_router.tsx#L59 . Here the mounter.appBasePath is checked which contains the base path of the application ie. /apps/kibana/login. Hence no route is drawn and the login form not shows.

[majagrubic]

Hi @g-w, thanks for reporting this issue.

[elasticmachine]

Pinging @elastic/kibana-platform (Team:Platform)

[joshdover]

@pgayvallet do you know if this is resolved by any of the fixes you've done in this area recently?

[pgayvallet]

This is resolved by #65977 + #67125 in 7.9+

Should I backport them to 7.7 / 7.8 ?

[joshdover]

Should I backport them to 7.7 / 7.8 ?

Yep, let's do that.

[pgayvallet]

#65977:
7.8: #68072
7.7: #68073

#67125:
7.8: #68089
7.7: #68090

[pgayvallet]

Will be fixed in 7.7.1 and 7.8.0+