Join
The Join 
tool combines separate line and arc segments into
a single connected polyline. It is the main tool for rebuilding a usable contour from
fragmented geometry — for example after importing a drawing whose outline arrived as
many disconnected pieces.
Joining segments
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Select the Join tool.
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Click a segment to start the result polyline.
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Click the next segment to add. Pix trims or extends the segments at their intersection as needed and shows a live preview of the joined result.
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Continue clicking segments to grow the polyline.
When the joined segments form a closed loop, the result is completed automatically.
While joining:
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Hold kbd:[Ctrl] and click to finalise the current polyline and start a new one.
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Hold kbd:[Shift] and click to undo the last segment you added.
The finished polyline is created on the default layer, and the original segments that were joined are consumed.
Demonstration
The video below shows several separate line segments being joined into a single polyline: