This lockup occurred on two consecutive attempts to edit this spec:
DBMS_REFRESH.SUBTRACT ( name IN VARCHAR2, { list IN VARCHAR2, | tab IN DBMS_UTILITY.UNCL_ARRAY, } lax IN BOOLEAN := false);
Was able to edit it down to this before the editor window and Toad locked itself up.
This one recently came through support and it's logged. I will look at it this upcoming sprint so it will be resolved in beta ASAP. In the meantime, you can disable Auto Replace on the Editor | Auto Replace page in Options as a workaround.
FYI, this is still an issue in 15.0.13.690. Perhaps it will be fixed in the next sprint.
Our sprint started yesterday and ends on the 16th. I have it logged to fix, but I have other bigger initiatives on deck, first.