Rather than require users to open a separate sub-panel in the sidebar, why not put the filter buttons at the top of the View pane, for one-click access? Also, some (customizable) keyboard shortcuts would be nice.
Why would they be present at the bottom of the view/edit pane? They’re not relative to the document being displayed; they’re relative to the current location.
Also, the information density can be high in filters, e.g., the Tags filter. How would that be easily and accessibly displayed?
And what shortcuts specifically? It’s hard to consider undefined things 
I may be using the wrong term. I tend to view documents as a list (isn’t that the View pane?) with each document opening to the right when clicked (ie in Preview pane, using Widescreen). So I’m talking about putting the filter buttons at the top of the list, which seems to me more intuitive.
As for shortcuts, something to toggle “view all flagged,” “view all unread,” “view all labeled red” etc.
That’s the item list.
So I’m talking about putting the filter buttons at the top of the list, which seems to me more intuitive.
Can you provide a mockup as I can’t visualize what you’re imagining.
Well I can only do a very rough job, but ….
You can see I’ve just moved the panel of filter icons from the bottom of the left sidebar to the top of the items list, just below the main “breadcrumbs” header. So if I wanted to see all flagged items in the list I’d only have to click on the flag icon. It’s just removing that extra step of having to open the info panel.
Thanks for the mockup. We’ll consider your suggestions.