fix: evaluate zoom-dependent symbol sizes at the drawn zoom

[mondsichtung]

Fix labels briefly drawn too large when zooming out fast with a zoom-dependent text-size/icon-size

On a fast zoom-out, tiles kept on screen from the deeper zoom draw labels at the size baked for their own zoom: camera sizes were clamped into the two stops around the tile's zoom, a restriction only composite sizes need (their per-feature sizes are baked into vertex data).

camera sizes now keep the whole stop table (plain arrays, still worker-transferable) and are evaluated at the drawn zoom, capped at the size the tile's collision boxes were built from. composite, constant, and source are unchanged.

Design: evaluate camera symbol sizes at the drawn zoom

Symptom

Zoom out quickly on a style where text-size or icon-size grows with zoom, and labels flash too large for about half a second. The tiles kept on screen from the deeper zoom draw their labels at the size for their own zoom, not the camera's.

Background

A symbol bucket classifies its size property into one of four kinds. The data is baked in the worker, so it must be plain transferable values, not the expression itself:

kind	depends on	baked at layout time
constant	nothing	one size
source	feature	per-feature size, in vertex data
camera	zoom	sizes at the two stops around the tile's zoom
composite	zoom + feature	per-feature sizes at those two stops, in vertex data

At draw time, evaluateSizeForZoom turns the camera zoom into shader uniforms.

Cause

For composite, clamping the camera zoom into the tile's two stops is forced: each feature only carries its size at those two zooms, so blending between them is all the renderer can do.

camera has no per-feature data, but took the same clamp anyway, for symmetry (per an old comment). So a tile parsed past the function's last stop draws one fixed size at every camera zoom. That is the flash.

Fix

For camera, bake the size at every zoom stop instead of two (still plain arrays). At draw time, interpolate within the segment containing the drawn zoom. This reproduces the size expression exactly, for interpolate and step alike. One wrinkle: a step's first stop is -Infinity, which cannot be evaluated at, so its base value is sampled just below the second stop.

composite, constant, and source are unchanged.

Why the result is capped at layoutSize

Collision boxes are baked from the size at tileZoom + 1. If a retained tile drew labels larger than that, they could overlap their neighbours, trading one artifact for another. So the drawn size is capped at that baked size (layoutSize).

The cap never binds inside a tile's own zoom range, so steady state is unchanged. It does not bind on the zoom-out this fixes either, where the correct size is the smaller one. The one case it leaves as-is (today's behaviour, not a regression): zooming in on a retained parent tile still draws labels slightly small until the children arrive. Fixing that means scaling the collision boxes, a separate change to the placement path.

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Generated-By: Claude Fable 5 (high)

[codecov]

Codecov Report

❌ Patch coverage is 65.51724% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.95%. Comparing base (ef90e66) to head (1632f31).
⚠️ Report is 5 commits behind head on main.

Files with missing lines	Patch %	Lines
src/symbol/symbol_size.ts	65.51%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #8175      +/-   ##
==========================================
+ Coverage   93.92%   93.95%   +0.03%     
==========================================
  Files         290      290              
  Lines       24916    24924       +8     
  Branches     6575     6576       +1     
==========================================
+ Hits        23402    23417      +15     
+ Misses       1514     1507       -7     

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[HarelM]

I think I understand the fix, but it would be helpful to show a short design document to better explain this change.
Also, please extract some methods out and move the in method comments to be tsdoc comments.

[mondsichtung]

Refactored in 1632f31, design document was added to the initial post