Tried to report this as a bug a github 
In opendistroforelasticsearch-kibana 1.12.0, the code pane is rendered too small to see the code if there is an error in the timelion script the first time you open a timelion graph in a particular dashboard.
Circumvention is to place cursor in the pane and navigate blindly with down_arrow down to the code line and the do a ctrl-a ctrl-x to get the code. Correct the code in a external editor and paste i back, upon which the code pane is rendered with a good size.
This is remembered in case of subsequent syntax errors in the same dashboard. Reintroducing the bug and logging in in a new browser the size of the pane still comes out fine.
So the bug is probably in the initial behind the scenes conversion from old to new builtin timelion style.