The goal of this issue is to improve the Segment auto-completer and improve it so that it suggests more values in the dropdown and works faster.
How it currently works
Currently only the values found in any of the last 500 visits will be used. But some segments are rarely used and none of the 500 visits would have it. For example imagine a custom variable that is set only on 1% of visits. When user creates a segment he may not even see any suggestion for this custom variable because only last 500 are used.
How it should work - Maintain mapping Segment <> API report - Use pre-processed API responses as suggested values. - automated tests for each segment dimension
see also DeviceDetection segment editor auto suggested values should show full strings #6772