No splitter appears when "visual/Source" property value is selected as default editor tab preference.
After some changes to the preference the Editor has stopped to open successfully (Divide by Zero error dialog appears each time I start the editor).

Test case:

EXECUTE: Run the JBDS for very first time (After workspace cleaning)
EXECUTE: Open some xhtml file in JBoss HTML Editor
ASSERT: The only text part of editor appeared on the screen (In my case "Visual./source" value was the default for default editor tab preference)

EXECUTE: Change the value for default editor tab preference and try to open and close JBoss HTML editor
ASSERT: Editor opens successfully, the splitter appears correctly depending on the default editor tab preference value.