[x]):When I expand file in the compare code view, it doesn't work, but at a review code view, the expand file is just fine.
When I inspect element, I find the web report a error "ReferenceError: Can't find variable: SimpleMDE" at the jquery.min.js:1386
...
Currently (1.12.0+dev-211-g316db0fe7) it's not working for expansion of short chunks in the review page either:

The same problem exists in 1.12.1 - I'm not sure if the root cause is still the same.
Upon loading the page I get:
[Error] Failed to load resource: the server responded with a status of 404 (Not Found) (tribute.min.js.map, line 0)
If I then click the expandy icon I get a standard HTTP auth popup and after disbanding that I get:
[Error] Failed to load resource: the server responded with a status of 401 (Unauthorized) (undefined, line 0)
On MacOS 10.15.5 / Safari 13.1.1
I think this is fixed already. OP please reopen if not.
Most helpful comment
The same problem exists in 1.12.1 - I'm not sure if the root cause is still the same.
Upon loading the page I get:
If I then click the expandy icon I get a standard HTTP auth popup and after disbanding that I get:
On MacOS 10.15.5 / Safari 13.1.1