Entry area and Send buttons are sensitive while access mode is read-only
When the port settings define the access mode as 'read-only', it is possible to enter text in the entry are and click the Send button.
This has no effect on serial port, but this is misleading as if you don't pay attention to the access mode you might think something is on the other end of the port (this is my own experience).
Moreover, a side effect is that the fake sent bytes are decremented from the TX count, leading to a big number displayed:
A solution (already implemented in my fork) is to set the sensitivity of both entry widget and send button to false when the port access mode is read-only.