How to bring focus to Show Fonts dialog (eg. in TextEdit) on a MacBook Pro keyboard?
After you open the Show Fonts window, you can press Ctrl+F6 to bring focus to the floating Fonts window. To return focus to the window you were in, you can press Ctrl+F4.

On a laptop, you'll also need to hold Fn when you press the above keyboard shortcuts unless you go to System Preferences > Keyboard and check "Use all F1, F2, etc. keys as standard function keys".

After you move focus to the Fonts window, you can press Tab and Shift+Tab to cycle focus through the different columns and the search field.