Significant changes in a source message should invalidate all its translations

[eemeli]

In bug 1977144, a Fluent source message changed from

custom-avatar-alt =
    .alt = Custom avatar

to

custom-avatar-alt = Custom

without an id change or data migration.

Normally we allow source message changes with no impact on translations, as we assume that it's a typo fix or something similar, but a change like this is clearly not that.

We should have some heuristic that can notice a sufficiently significant change in a source message, and reject all of its translations when it happens.

To catch this specific case, adding/removing patterns in a Fluent message should qualify as "sufficiently significant", but we could go beyond that as well, and e.g. define some minimum edit distance in the source that would trigger rejections.

[mathjazz]

See also: #3500.

[eemeli]

This is a bit different from #3500, though, as that's about noticing better when entities don't change.

[mathjazz]

Agreed, left that here in order not to make the problem described in #3500 worse.