Why is it weird to yield a too-small window as the final result? That's what I'd expect, I think.

I'd expect the return type of .windows(3) to be an iterator yielding zero or more arrays of 3 elements each. Yielding an undersized window means you have to check for undersized windows before you can address elements in the windows, or always remember to deal with the elements being undefined. Programmers will most likely forget this.

Which is more likely, though - a mistake caused by not noticing that your iterator doesn't yield complete chunks, or, one caused by not noticing that a partial chunk was dropped?

This probably calls for an option, if the dropping behavior is needed.

I think the default should be that if you ask for a window of 5 items, you need 5 items to get a window.

In Kotlin you have to opt in to "partialWindows": https://kotlinlang.org/api/core/kotlin-stdlib/kotlin.collections/windowed.html

val partialWindows = sequence.take(10).windowed(size = 5, step = 3, partialWindows = true)
println(partialWindows.toList()) // [[1, 2, 3, 4, 5], [4, 5, 6, 7, 8], [7, 8, 9, 10], [10]] 

Having an options bag to choose a mode would align with https://github.com/tc39/proposal-joint-iteration, which currently defaults to "shortest", dropping values that can't be zipped with anything. With windows having an option to choose the padding may also be useful, i.e. choosing 0 instead of undefined.