Rain particle volume has a hard visible boundary when viewed from inside a building.
Consistent with the boundary of the camera-attached rain emitter volume, or of the under-cover rain suppression region, being cut hard rather than faded. Visibility depends almost entirely on background contrast — it's invisible against trees or town background and obvious against overcast sky and open water.
It was first noticed in a custom building with large, intact glass panels. That is likely why the issue is rarely seen: almost all vanilla windows are broken or empty, so players seldom get a clean, uninterrupted view of distant rain from indoors. The same artifact is present in vanilla buildings — it is just harder to spot through smaller openings against cluttered backgrounds.
A straight-edged region with noticeably reduced or absent rain is visible in the middle distance. The edges are hard, not faded.
Diagnostic observations
The boundary is camera-locked — it tracks with the player when strafing rather than staying fixed to the window or the world.
Its apparent height changes with distance from the window.
Its edges are aligned with the rain's fall direction, so they appear diagonal in wind.
It is not visible when standing outdoors in the open, only from inside looking out.
Reproduced in an unmodded building and modded building.