I love to minimalize an usage of mouse. Keyboard is my prefered input device. Thefore I recommend:
1) Provide better visual feedback which panel is active (e.g. selected/focused) when keyboard shortcut "Switch Focus to Next Panel" etc. is used. For example:
- draw the focused panel with bold framed or -OR-
- draw the only upper line of the frame with a bold line -OR-
- add color for "focused feed in active panel" (to the current possibility for "focused feed"). Similiar to news and somethoign new for browser -OR-
- use any other visual feedback.
2) Provide possibility to set keyboard shortcut to easy set active (eg. selected/focused) panel .... Tree, Categories, News, Browser.