It would be nice if typing in the search field for a category would automatically update the list on the page before pressing the "Filter..." button. I would find this very useful because during a game is the time I'm most interested in looking up rules quickly and extra typing and clicking just adds friction to that process. Also, if someone is typing into the search (or other filter fields) it's probably unlikely they still need to see the currently-shown list any more.

For example, one of my players might cast the spell "Compulsion"; currently I must do the following steps:

1. Click on "Spells".
2. Click the search field.
3. Type "Compulsion".
4. Click "Filter spells".
5. The list now shows just one spell. Click on "Compulsion" in the list.

A faster user experience might be:

1. Click on "Spells".
2. Click on the search field.
3. Type "Comp". The list already shows only "Comprehend Languages" and "Compulsion".
4. Click on "Compulsion" in the list.

A further enhancement could even be a universal search field which would reduce the steps to:

1. Click on the search field.
2. Type "Comp". The list shows "Comprehend Languages", "Compulsion", and "Helm of Comprehend Languages".
3. Click on "Compulsion" in the list.