Comment 4 for bug 166009

Union does a union of the areas, not necessarily a union of
the strokes, and not necessarily preserving direction,
thickness etc. of the strokes. It is not in general
possible for a single shape to embody the union of both the
areas and strokes of the input shapes (even ignoring
direction, stroke properties etc.). This distinction also
manifests itself with "self-overlapping" shapes (having a
region whose winding number is 2): the stroke around the
double-covered area is allowed to disappear.

A straight line segment has no internal area. As such,
doing a union of areas is allowed to discard that line
segment: whether the line segment is present or absent, the
shape has the same internal filled area.

One could envisage a slightly different operation where one
considered points on the edge to be part of the area. That
would be a feature request rather than a bug report; feel
free to re-open as such. (This wouldn't be enough to
prevent self-overlapping shapes from losing some of their
stroke.)

Use the Combine operation to do a union of strokes. This
seems to be what the submitter wants to do.

I suggest that the menu item should be called Area Union
instead (and similarly Difference, Intersection and Exclusion).