Discovered an issue with the photo browser today, here's the workflow:
Open a photo in the Photos app, choose the Pixelmator Photo extension
Click the + button to add a layer, and choose "Photos", the Photo browser will pop up, but it's tiny.
Photo browser is tiny when choosing to add a layer via Photos extension
2022-01-31 16:10:30
Great!
The photo browser is really the very weak point in this workflow, but I've figured out if I view the images in Photos ahead of time then it will download them from iCloud, and should appear at the very bottom of the photo browser.
2022-02-03 11:36:00
The images must be downloaded to the device's local library for Pixelmator Pro to be able to edit them, so this part can't be avoided, unfortunately. It would be nice, however, if there was a way to sort and filter photos in the Photo browser. We'd like for the future versions of the Photo browser to be able to do this.
2022-02-04 11:05:02
Pixelmator Photo is not a layer-based editor, so it should be able to work with images in the Photos library directly, without having to import them (which would require downloading them first). I'm not a developer myself but my guess is that Pixelmator Photo for Mac will download images from iCloud as you open them, just like the Pixelmator Pro Photos extension does right now.