I did it from Layout, so I could modify the line weight first.
Mmmmm, that’s odd. Attached are two screen grabs from my Layout.
The right hand side clearly shows little tiny portions of the vertical lines, whereas the left side the horzontal line is hidden because it appears to be sloping.
I made a new one from your SKP file. Note how I set the render type to Vector and the Lineweight to 2.0. Then I exported a PNG at 300 DPI and cropped it in an image editor to get this:
Then imported it as a new Pattern Fill. The SU viewport is on the left and the pattern fill is on the right. I scaled it to 0.50x but it could be scaled more to make it smaller.
In the last image you posted the 2nd line from the bottom is slightly off from horizontal. The red circle is around the tiny step I see. The black line in the middle is horizontal but there’s a step in the anti-aliasing suggesting that something is off.
However, when I export the file to PDF and open in Photoshop, it’s straighter than straight.
So I can only assume it’s a display issue.
I think I can live with it for what I need to apply these patterns to. In Layout, they will only be used to show Tile layout for a tiler. I can scale the patterns to suit the scale of the Viewport, and the main annotation will be to indicate the Datum point to start tiling. And, it PDFs ok anyway.
It was just one of those annoying little buggers that I wanted to try and solve.
I’ve come to the conclusion that you’re right @DaveR.
(It will be Apple, being more concerned with making sure people can have stupid shortcuts and gimmicky touch-bar malarkey, to quickly add emojis, rather than spend time on their screen quality )
It’s not your pattern and it’s not the shape you’re filling. It seems to be Layout. I tried with the built in Geometric Tiles/Black Linework/Basketweave and I see the same thing happening above a horizonal line drawn with the red inference in Layout.