Now hit cancel. You will see message "Cancel Requested" in Task Progress but the statement continues.
var n_count number; begin select count(1) into :n_count from large_table; end; /
No, SQL Developer always uses JDBC, it's just a matter of which version (the [thin-only] version it's packaged with, or the Oracle client install that overrides it) and whether we ask it to use Thin or Thick/OCI (via a preference, if a client is installed).
Ah, and one detail that can have its importance: I am not using JDBC to connect to the database (18.104.22.168 Enterprise Ed.). I use an Oracle 22.214.171.124 client for windows 32 bits, on windows 7 Enterprise SP1.