The recently added option to split the sources panel horizontally does not play well with the button that shows/hides the sidebar. The proposed solution:
1. Hide this button when the "Split horizontally" option is checked.
2. Force-show the sidebar if the sidebar is hidden and "Split horizontally" is clicked.
Created attachment 188861 [details]
Created attachment 188867 [details]
Committed r143227: <http://trac.webkit.org/changeset/143227>