I think I saw this long-standing request mentioned again recently but can’t find the thread. +1 here in support. I wonder if the activity on 3.3+ might include this request? Meanwhile, I think the only way for a user to tell that an action is in progress is to try doing something else in the module. Clicking on the chat input box seems the safest.
A couple of things aggravate the requirement. Firstly, action button-clicking seems to be an inexact science - even when the action button has a been accurately defined (a challenge in itself), so often (I find) a button press does not get actioned until a second or even third click. Secondly, Vassal performs an entire operation end to end before any feedback is possible. So, for example, once a trigger is activated, everything in that trigger must complete before any output is produced. So there is no way for a module to provide even a starting message (it will appear, but only at the end). Except, of course, to break the automation down into chunks, each of which would require further user action to proceed.
One final possibility is to use Alert() in the trigger, but that will hold up the entire operation until the user acknowledges the Alert (which may be covered by other windows in the module).
I’d be happy to learn if I’ve misunderstood any of the above.