The left mouse button is used to draw lines and to place the pivot (click without moving the mouse).
The right mouse button is used select lines. If the button is held down it is also possible to move lines until releasing the button. If there is no selection then the BoxSelection will be entered.
When double right-clicking on a line the selection will grow to the connected lines.
A tripple click will select all connected lines until a certain size is reached.
The mouse wheel is used to zoom and when pressed to pan the view.
Line creation modifiers
Hold the corresponding key and then create lines.
Create multiple lines
Temporarily toggle grid snapping
Use line just for cutting other lines at intersection points
Create guide lines with the left mouse button. Measure with the right mouse button
Free drawing mode
Mousewheel scrolling modifiers
Change selection radius
Change line thickness (also changes selected lines)