When opening a subfolder in folder view it disappears when the cursor touches the subfolder view. That makes it unusuable. Reverting bfee2b093a2e3f926f9d6de59fae5c64dedb7cf8 '[Folder View] Clear hover state when mouse leaves view' (https://phabricator.kde.org/D3593) fixes it.
*** Bug 376689 has been marked as a duplicate of this bug. ***
Can you test with 5.9.x? The relevant code has changed a lot and this is probably fixed (I can't rep it on master).
*** This bug has been marked as a duplicate of bug 374291 ***
This was also fixed by e80a4b87cfa9870, but there's no 5.8.6 with it yet.