When you have 2 stacked splits both with the same config and the config does not have any UUIDs then the config of the lowest priority split overwrites that of any higher priority split instead of merging the 2.
The problem is that the key of the sequence dos not correctly identify the element. For elements that have a uuid field we use that but this doesn't work in a generic way.
Split congif in 2 stackable split that as no UUID
In ConfigPatchMerge we check the config schema to see if there is an index callback specified and use that to generate the index when transforming the sequence indexes.
Modules can alter the config schema to specify this callable. This is probably much more efficient than dispatching events or call alter hooks. But it is a new extension mechanism not many developers are familiar with. Maybe some profiling could be good.
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API addition: a new config schema property is checked on sequences.
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Fixed
2.0
Code
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