Select the root node (Classes) and press / (separate the character) on the numeric keypad. It will ruin everything. Then press the + (plus) key on the numeric keypad, and the first level will expand.
You can also play with - (minus, collapses everything except +, will expand everything, not just the first sublevel) and * (multiply, expand everything).
The left arrow key works the same as the right arrow, works the same as +.
Windows (, ) Windows, .