Comment 2 for bug 1598999

After some more tests, I narrowed down the possible cause to the 'Show property panel' setting in IBus Preferences. Enabling IBus with 'Show property panel' set to 'Do not show' resulted a working task bar panel which responded to mouse clicks. Setting it to 'Always' made the task bar panel stop responding to clicks.

For those input methods show a property panel, the task bar panel stopped responding to clicks when the property panel was visible if 'Show property panel' had been set to 'Hide automatically'.