The sequence order in Statements - Log suggests the ALTER occurred before execution of the SELECT, so the cancel attempt had nothing to do with that.
Perhaps I imagined this, but I thought I cleared the Statements - Log and ran "DESC DBA_EXTENTS" (on a user with appropriate privileges). The first time the log showed the "DESC" command triggering the ALTER, but when I tried to repeat it in order to get a clean screenshot, it did not. Probably did not pay careful attention to what I did.
What version of SQL Developer do you use?
there is a bug (fixed for 20.2) where doing a DESC on an invalid view, we would compile the view in the background to get an accurate column list for the desc
Good to know I'm not crazy. Thanks!