REGRESSION: UI: Not all folders are shown when the list is long

Reproduction path:

• add a lot of folders or open a lot of them
• use firefox (i didn't test with other browsers)

Result

• see the full height is not used, see the arrow in screenshot

Expected result:

• the full height is used

I think the problem lies here:
hippo-tree has a static: 350px height, while hippo-accordion-unit-center has a variable height