This is also a big problem for me and NOT fixed by the above.
You have to double click on each top level keyword in order to close the tree leaving top-level keywords visible (like v3)- this is very difficult when there are a large number of top level (level below root) items. In Jabref 3 the right click menu had a collapse sub-tree item which at the root level (All entries) left top level items not collapsed. This was the most useful state.
In V5.0 double click on the “All entries” item collapses everything which is not what I want - there is no way (as far as I can see) to expose just the next level items as another double click re-expands the tree again with all the sub items expanded as they were before it was closed.
This is a show stopper for me - I’m still using version 3.2.