Question: can the BlockGasMeter be used for absolute tx position?

[joshklop]

The amount of consumed block gas is used here to uniquely identify the position of a transaction in a block.

Is it possible that a transaction could consume zero gas, leading to a collision (that tx and the tx that follows will have the same "absolute index")?

[chipshort]

In theory that is possible, but a zero gas transaction is inherently problematic, since it can be used for spamming and potentially a DoS, so transactions shouldn't be zero gas in the first place.
Also, this is just used for metadata on the contract history, so it's not critical if this were to happen.

[chipshort]

Looks like a better method to get a transaction index was introduced at some point.
Let's use this as an opportunity to explore switching the contract history over to that too.