Lack of documentation on how mapbox-gl merges lines for label placement purposes
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Since Jul 9, 2021.
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Description
We've come across an issue where gl-js seems to (from our perspective erroneously) merge lines before placing labels. This results in labels for feature A being placed along feature B, given both have `symbol-placement: line` and identical `text-field` values.
The screenshot below illustrates the situation. The label on the left belongs to the feature on the right (color is being set via `feature-state`, using the features' `id`s). The two linestrings meet exactly at the point where the building in the middle is placed:
I have found [this comment](https://github.com/mapbox/mapbox-gl-js/issues/3645#issuecomment-262414642) which seems to indicate the heuristic for merging is based on data-driven properties(?) but I've failed to find any further documentation on how exactly this works.
**mapbox-gl-js version**: `1.13.0`
**browser**: Chrome 90.0.4430.212 (macOS 11.1)
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