Dear Xara...

This has been a forever Bug.

With the Shape Tool, click and then Ctrl+Drag to finally click for a Horizontal or Vertical straight Line.

Switch to the Selector Tool.
Either the H or V will have a zero entry.
Bug - try changing the other positive entry to a different positive value.
You should get an Error message saying you cannot.

If you alter the Line width to a large number, you will note that the developer agrees that a Line has zero height or length (matching its Perimeter) so a zero entry is a valid input.

How many are aware that you could switch to the Shape Tool and adjust the line length there?

Please rectify this Bug.

Bug - for a Zero Line width Line, when you Convert Line to Shape it simply vanishes.
Yet, adding Shapes with Lines of any Line width and a Shape replaces the union part of the Line and Shape with a combined Shapes that has been given to topmost Line's Line width.
Bug - if this Line has a zero Line with the resultant Line fragment is now a Shape with an interior size of 100 millipoints.

This means when I combine Lines I will always have this error added in.
Why not 10 mp?