I can get the midpoints aligned, as shown in the screenshot, but I want it to be anchored to the horizontal line so that when I change variables (angle, length) it will pivot or stretch symmetrically with its midpoint staying on the horizontal line or its midpoint.
I’ve found some similar posts but they only show how to align the midpoints, not constrain them, and many of them were old and seemed outdated, or had broken links/missing videos.
Il suffit de bloquer ici le segment horizontal avec la contrainte cadenas.
If you cannot select the center purple dot and lock it. Maybe you could draw a short segment (construction line) ending at the center of both lines. That should give you a point to lock in the constraints?
At present you cannot constraint 2 midpoints to be coincident without using a third (construction) line ending at the mid points, as @Oregonerd suggested, because midpoint constraint works only with end point : you select an end point and then a line and then apply midpoint constraint.
The other way is to draw two lines, starting at the mid point of the horizontal line, and set 2 constraints on both of them:
