Sage 5.6, Ubuntu 12.04 LTS, Firefox 19 or Opera 12.4
After an update (see above) a click on %hide doesn't expand the cell for editing.
This bug is still alive in sage 5.10
One of my department lecturers made some noise about this issue and asked me to have a look.
It appears Jason missed binding the cell_focused function to the onfocus event for any %hide input cells.