Double-click the caption to shrink the window to the title bar and a launch field. Type a few letters to list installed Start menu programs and executables on %PATH%; Enter launches the selected one. Esc or another double-click restores the window.
One process for all windows
Starting Sumatra File Manager again opens a new window in the program that is already running, on the current virtual desktop, instead of starting a second copy.
Whitespace, soft wrap and Edit in the viewer
The text viewer and editor can show whitespace characters and toggle soft wrap. The viewer has an Edit button (F4) that switches it to the editor.