kylin website - Left navigation column of Docs/Development is missing if width of browser window get small

Open Chrome/Safari and navigate to:
http://kylin.incubator.apache.org/development/
http://kylin.incubator.apache.org/docs

Actual: Left navigation column is missing if width of browser window get small.(See attachment)
Expect: There should be a navigation list for user to navigate between links, no matter how browser window size changed.