I'm using your demo to reproduce the issue.  

When the user enters a value into the categories and tabs, the focus is not placed in the products field, instead it goes to the View Order button.  This makes using the cascade "feature" very difficult for users.  The focus should go products so they can further refine the search.  And then finally to orders.  But each time they tab they have to move the cursor back to the next field.

Also, since the product and order fields are disabled it prevents user from entering data (even if they know the correct value).  Is there a way to leave the cascade functionality whilst leaving the "child" fields enabled?  We were using it in a Region, Country, City filter, but because of these issues we have had to remove the cascade.

Expected behavior would be to leave all the fields enabled, if the user selects a category, then the subsequent fields (products and orders would be filtered to the selection) so that when the user starts typing in those the parent filter is added to their filter.

Whilst we are discussing filters, why if the filter is narrowed to a single item is it not automatically selected?  Currently the user has to filter, then select the item and then tab.  We have had numerous "bug" reports raised because the user didn't select before tabbing even though the item is the only one in the drop-down and appears to be selected.

Similarly the Mutliselect filter allows typing values, and even when the user selects the value in the dropdown and is added to the selected list, the typed text remains, making the mutliselect field value invalid on submission. 

Repro:

Open page: CascadeComboBox
Move focus to Categories combo (i used my mouse)
Type 'p' which opens the combo and shows 3 items starting with p
Select 'Products' using the down arrow (field now displays Products)
Click Tab
Focus moved to View Order button and NOT Products field.
Using the mouse to select a value works fine, but our users don't want to have to use the mouse as this slows down data entry.