The nine coordinate helpers on GameWorldView were the first seam of the
planned decomposition: stateless math that every draw method, the placement
validation, and the belt-drag code reached into private state to get at.
WorldCoordinates is an immutable value built from the viewport size, the world
height, and the scroll center. It lives in lib/core rather than ui, following
BeltDragPath and TunnelCompletion — UI-only helpers kept there so Catch2 can
reach them. That is what lets the transform math be unit-tested at all, since
the test target links lib only.
Rather than leave delegating one-liners behind, the snapshot is threaded
through the call graph: paintGL builds one per frame, every world-space draw
takes a const WorldCoordinates&, and the screen-space draws (vignettes, replay
overlay, debug text) take none. Those methods no longer touch m_scrollXTiles,
width(), or height(), which is most of the groundwork for extracting
WorldRenderer later. The mouse handlers each take their own snapshot.
The snapshot is deliberately not cached in a member: a resize or a scroll
would silently invalidate it, and a stale transform surfaces as misaligned
hit-testing rather than as a visible failure.
widgetToTile and widgetToWorld had the same arithmetic inlined separately;
the former now goes through the latter.
The architecture doc's coordinate section was already stale — it described a
painter.translate approach and a hardcoded tilePx = 20, neither of which was
true — so it is rewritten to match the code.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JcReq7hVk4KUPhTDKWAG7K
