I decided that I needed to make a pair out of squared fabric. This would make the problem immediately visible. I bought some inexpensive, Rupert-Bear-esqe fabric from Coalville Fabrics, and looked at it for a few days.
And then I did it.
The leggings pattern I'm using is a self drafted one from my SureFit Deigns trousers blue print. There are two pieces per leg (rather than 1 piece that you get when you buy a leggings pattern from, say, Patterns for Pirates or Halla or LoveNotions (I have kids leggings patterns from those). I had tried the P4P adult leggings, but the shape just didn't suit me
Normally, I cut each pair of pieces in one go, with the fabric folded. This saves time, and it means that when I put the two legs together and sew the crotch seam, the two fronts ar identical and the two backs ae identical. Where pattern matching is needed, this approach doesn't work.
So, I had to cut each of the four pieces separately. I'm using stretch fabric, of course, so this also added a little extra complication. It took a lot of time and patience, and involved quite a lot of rude words, but eventually I cut my pieces. I also identified what was causing my problem, even before I'd done any sewing, so that was a bonus.
I basted one leg together, and I was amazed to see that I'd got the pattern matched on both seams! I did the second leg and discovered a missing bit of fabric. I couldn't face trying to cut another whole piece, so I decided to patch it. It worked. Then I decided to overlock the seams, before I actually joined the two legs together.
And then I pinned the two legs together, and kept my fingers crossed. Well, kept them crossed in my head. Obviously.
The pinning looked good, so I went ahead and sewed. And I was amazed.
I did a happy dance. I showed DH, having briefed him on exactly what was amazing about what he was about to see, and letting him know exactly what sort of reaction I was looking for.
I tried to take pictures, but that proved a bit of a challenge. Here are a couple of them. Please note that I hadn't put the waistband on, nor had I adjusted the length of the legs.
But look! All the seams match. The two legs are even, front and back.
Will I wear them outside? Nope. I won't even wear them in the house. I'm just going to keep them as a reminder of an achievement.