Spark does not display properly when run in http://localhost:8888 in MAMP PRO

If I serve Spark from any directory other than http://localhost:8888 in MAMP PRO, it displays perfectly fine. However, if run from http://localhost:8888, it will not render properly.

See:

http://imgur.com/a/eyPN4

My env.:

APP_NAME=Laravel

APP_ENV=local

APP_KEY=base64:XXXXXXXXXXXXXXXXXXXXXX

APP_DEBUG=true

APP_LOG_LEVEL=debug

APP_URL=http://localhost:8888

DB_CONNECTION=mysql

DB_HOST=127.0.0.1

DB_PORT=8889

DB_DATABASE=spark-in-htdocs

DB_USERNAME=spark-in-htdocs

DB_PASSWORD=secret

...

My app.php:

...

'url' => env('APP_URL', 'http://localhost:8888'),

...

My document root in MAMP PRO:

file:///Applications/MAMP/htdocs/spark-in-htdocs/

http://localhost:8888 pointing to above document root

Why can't I serve Spark from http://localhost:8888 ?