TL;DR: the Affinity Designer Knife tool currently does not handle open curves that have a fill, and using the Knife tool on an open curve results in smart corners being converted to Bezier curves. These limitations should either be listed in the Knife tool's online documentation (which currently says merely, "The Knife Tool lets you draw a freehand or straight cutting line across any open curve or shape to split the object up into smaller fragments") or the app should be updated so these limitations no longer apply.
Longform:
I'm using Affinity Designer to create a jigsaw puzzle (to be cut on a laser cutter). This involves a number of open curves. Eg:
I'll sometimes need to "chop off" lines - eg, to do a test-cut of the 3x3 grid of puzzle pieces above.
It seemed like the Knife was the correct tool for the job - other image editors I've used also have some variant of "cut stuff" tool, and the documentation says it can be used on open curves. But when I made a simple, single cut below the 3x3 grid, I got this as the result...
...which is not a particularly intuitive outcome.
I eventually managed to find this post, which says that the Knife tool won't work on curves that have a non-transparent fill. I changed the fill on my curve to transparent, and boom, the Knife tool worked as one would expect - except that all of the smart corners (used to make the puzzle bulges) were replaced by Bezier corners.
In an ideal universe, I'd like the Knife tool to work as I'd first expected it to, but at the very least, I think its current limitations should be mentioned in the documentation - it seems an easy way to save many others the wasted time that I went through trying to figure out what was going on.
(PS: I realize there are other ways than the Knife tool to accomplish what I want, and have been using those - they're more tedious/repetitive, especially since there doesn't seem to be a keyboard command for Break Curve, but they do work. I still think the Knife docs should mention the tool's limitations.)
