Using the minimum scale size with layout/classic scrollbars is not supported and buggy. Making it work is somewhat hard and not very important since it's minscale is more important for mobile where there are no layout/classic scrollbars.

This bug is to fix this so it doesn't happen in our tests. Ways to fix

1. set ui.useOverlayScrollbars pref in any test where this happens
2. disable the min scale code if we don't have overlay scrollbars

Here is a try push where I assert anytime we use the min scale size and the size is different from what we would have done and we have layout/classic scrollbars.

https://treeherder.mozilla.org/#/jobs?repo=try&group_state=expanded&revision=47ffe3d22f960ad3a1a1513acd380fc777c62089

There are several failures that are unrelated in that push, so just ignore anything that isn't a fatal assert.

Here's a patch I use to assert.

Do you intend to land this patch (or some variant)? It would be good to do so to catch further tests getting added that trigger this condition.

We need to fix the tests first (or our code to ignore min scale with layout scrollbars). I only fixed the reftests. There are some other failures that I haven't looked into at all (because the tests don't fail with my desktop zooming patches).