In the Schema Browser, I can bring up data grid that column is CLOB cell (HUGECLOB).
In Toad for Oracle 11.5 I can see the flashing cursor when double clicking on cell.
In Toad for Oracle 11.6, no cursor is displayed. I can see the cursor position with the "T" in the in the column scale, but I have no idea what line the cursor is currently on.
Click off the modal Grid popup editor window (by switching to another application returning to Toad for Oracle ) one can bring the flashing cursor back to the Grid popup editor window.
Waiting for fix in a future version of Toad for Oracle.