I've tried dojox/mobile/tests/test_ComboBox.html
on Galaxy S3
Android 4.1.2
using Dojo 1.8.3
, and I did reproduce the issue you encountered with both Chrome and the platform browser.
On the other side, with both browsers it seems to work using the Dojo 1.9.0 rc2
(http://download.dojotoolkit.org/release-1.9.0rc2/). So for now I would recommend upgrading to Dojo 1.9
if possible in your context.
If upgrading is not an option for you, I'd suggest that you open a ticket at http://trac.dojotoolkit.org.
Side-remark, dojox/mobile/ComboBox
is flagged "experimental".