Comment 5 for bug 1613708

I can think of two more reasons why buttons would become unclickable:

1. One of your modifier keys (Alt/Shift/Ctrl) are either physically or logically (in software) stuck down. Try tapping all the Alt/Shift/Ctrl keys. This is definitely a problem for X apps under the Mir demo servers after you Alt+Tab but not a problem under Unity8 AFAIK.

2. Try rotating the tablet to a different orientation. You might be experiencing a problem similar to bug 1502805.