Controls

Left drag

Use the active tool

Ctrl+click

Toggle an object's selection

Ctrl+drag

Box- or lasso-select on empty space (curve back to switch to lasso); selects objects by center

Scroll wheel

Zoom in / out

Ctrl+A

Select all objects

Delete / Bksp

Delete selection, or remove the last polygon vertex

Right click

Add final vertex and close the polygon

Enter

Close the polygon

Esc

Cancel the polygon or shape in progress