You can use the Midpoint constraint to connect an endpoint with the center of a line. This constraint ensures that the connected endpoint remains at the center of the line even if the length of the line changes.
Select Midpoint from the constraints menu or drag an endpoint onto the center of a line.
Learn more by watching the video below:
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Endpoints that are connected to elements with a Midpoint constraint are shown with a filled center. For more information, visit States of sketch points.