Fixes #1501.

The bug

Depending on the hash of a scoped classname, a selector such as && would end up transformed into a selector that was not scoped to the classname represented by &, which would cause an invalid selector error.

An example from the test case I added involves the local classnames debugName_hash1 and debugName_hash1d. The && selector would become debugName_hash1debugName_hash1, which contains debugName_hash1d as a substring.

When this occurs, the local classname search returns 3 results of the form:

[
  [14, ['debugName_hash1']],
  [15, ['debugName_hash1d']],
  [29, ['debugName_hash1']],
]

Note that we iterate over these search results in reverse order. In the above case, the replacement for the middle element would overlap with characters already replaced by the last element, which causes the bug.

It might not be obvious as to when this specific set of classnames would ever be generated. That is, a classname that both contains a substring equal to another classnae, and both starts and ends with the same character.

As mentioned in #1501, the refCount appended to a classname hash increments by 1 for every classname generated in a file. This value is encoded as a base 36 value, so it goes from 0-9a-z, then 1(0-9a-z), etc.

This means that once you roll over to the a-z range, you'll eventually end up with a classname that satisfies both of the constraints mentioned above. Ideally you wouldn't need to write a selector like && as increasing specificity shouldn't be much of an issue if you're using scoped classnames, but regardless this shouldn't break selector transformation.

The Fix

The previous logic of checking startIndex >= lastReplaceIndex didn't cover this case. By ensuring that we only ever do a replacement when lastReplaceIndex > endIndex, the replacement will only affect characters that have not already been replaced. We actually want to negate this logic as we continue when we don't want to replace. This is how we end up with lastReplaceIndex <= endIndex as the skip condition.

To test out this fix, install @vanilla-extract/css@fix-buggy-selector-transform.