When the VarStore.optimize() algorithm was improved with the addition of a priority queue where pairs of encodings are sorted by decreasing gain (from merging one into the other), specifically with this commit https://github.com/fonttools/fonttools/commit/47ec18f788, the pre-sorting step of the todo list (before the queue itself was populated) was kept to make sure the algorithm produces stable results no matter the order of the inputs.
The optimizer's output was itself sorted, again for stability, but using a different key function with_sort_key() from the one used on the input todo list gain_sort_key().
The rationale for a distinct gain_sort_key, where encodings get sorted by maximum theoretical gain (decreasing, initally, when reverse=True was set, then incrasing as reverse was somehow dropped), is no longer needed now that a priority queue is used (which sorts by actual gains from merging specific pairs of encodings) and is a remnant of the previous algorithm.
I propose we keep some pre-sorting to ensure stability (in case the priority queue initially contains multiple pairs with the same gain), but we use the same width_sort_key that is used at the end.
Note this doesn't change the overall level of compression achieved by the optimizer, but makes the algorithm a bit less complicated, and easier to match in our alternative Rust implementation.
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