Steps to reproduce:

a) download http://reports.qa.atl.jboss.com/binaries/RHDS/release/2.0.0.CR1/200812221144/installer/build/jbdevstudio-eap-linux-gtk-2.0.0.CR1-R200812221144.jar from http://reports.qa.atl.jboss.com/binaries/RHDS/release/2.0.0.CR1/200812221144/index.html
b) install using defaults
c) launch
d) create a seam project: you'll get a .css file in $projectName/WebContent/stylesheet/theme.css
e) open theme.css (double-click)
f) note the new toolbar button. Click it to launch the CSS Dialog
g) make changes to your active CSS doc, or just hit OK
h) note the new toolbar button has vanished
i) close the .css file and reopen. Toolbar button reappears.

(Note that you need not create a seam project – all you need is any .css file.)

Question: shouldn't it just stay there the whole time, rather than appearing/vanishing unexpectedly?