Pattern dimensions don't match grid

[Greg dJ]

Small thing, but it bugs me: how can I get predictable results when I want to apply a precise pattern pixel size?

For example, if I create a square pattern and set it to 50 pixels, and enable a 50-pixel basic grid, the pattern must be just a sliver larger than because the pattern and the grid do not stay in alignment - the pattern appears to be ever so slightly larger over the course of a few dozen grid lines.

Is there some trick where I have to adjust the file settings (resolution?) to get these to cooperate? Is this a bug?

[Brian_J]

I’m guessing the issue is with where the elements in your pattern are placed. If you can provide the pattern file, someone can take a look. 

[Greg dJ]

Thanks, @Brian_J - your comment jogged my mind in just the right way, and I think I'm on track now:

• I had originally designed the pattern at an arbitrary size, then scaled it down to the desired 50px.
• On closer inspection, I realized that it was being antialiased such that it had ever-so-slightly more than 50px per side, too small to register in its dimensions but over the span of a few iterations turned into the issue.
• I tried rebuilding the pattern at 50px then the pattern layer conformed to my 50px grid.
• I also tried building the pattern at a multiple of 50px, then rasterizing, making pattern layer, and scaling it down, and this also works.
• During the above I tried turning off antialiasing but that doesn't seem to matter as long as the pattern ultimately has exactly those 50px dimensions.

This was also helped by my previously learning about how Affinity likes to place things (incl. artboards) at subpixel locations, which can throw off export dimensions.