Today I found an inconsistency in the order you have to select sketching curves for the offset tool.
The inconsistency is between selecting ordinary sketch lines and construction lines (aka. closed shapes like an ellipse)
Sketch lines:
- Activate the offset tool
- Select a sketch line
- Offset is generated

Construction lines:
- Activate the offset tool
- Construction lines cannot be selected

... Workaround:
- Select the construction line first
- Activate the offset tool
- Offset is generated
