Steps to reproduce the issue

Select System > Templates > Site Templates > open Cassiopeia
Click on a folder icon

Expected result

The folder/directory tree should open, as it does in J3.

Actual result

The tree does not open. Only if you click on the adjacent word, e.g. 'css' or 'html'.
Instead, in Firefox, it highlights the whole link with a blue box:

It doesn't work in Chrome, either, although there is no blue box. In both cases, the blue becomes darker on hover.

System information (as much as possible)

Tested on last night's backend Dev nightly installation and also the main Alpha 11 Nightly from 9th August.

Additional comments

On a side note, it would be even better if the file icons were white rather than blue, to differentiate them more obviously from the folder icons, as they are in J3: