I finally found the source of this oddness after a couple of years . . .
On startup in development environment, there is a check that updates the
main stack if it is not from today, using "save as". The name is to the
effect of "mcp.171004a.livecode"
This invariably triggers a live code message asking what to do with the old
stack (and sometimes a flurry alternating between the old and new stack as
I save each).
There is also a custom dialog that automatically loads and asks for the
database table to open (with client names, not raw table names).
The second of these to launch blocks the first. The result is that I have
to hit "cancel" on my own dialog, but execution remains stopped until I
deal with the system message. In turn, that means that the dialogData
becomes "save" rather than the "cancel" associated with my own dialog.
It never occurred to me that the system portion would be using this
property. (To add insult to injury, I changed from setting a custom
property in my dialog to using dialogData).
I suppose that since the IDE tries to be written in live code as much as
possible this may not be a bu, but it is certainly unexpected . . .