Use instancing for compact hexbin rendering

[Alek99]

Closes #175

Summary

Replaces the client-side six-way hexbin fan expansion with a dedicated instanced renderer:

• one instance now uploads only encoded center x/y plus one color scalar
• HEXBIN_VS generates the six triangles from gl_VertexID, using the same ring as the Python renderer
• the existing mesh fragment shader remains responsible for LUT, alpha, and barycentric edge coverage
• VAO, retained-payload rebuild, context restore, and resource cleanup paths cover the new program and compact buffers
• the wire contract is unchanged: centers plus one scalar per occupied cell

Hexbin remains intentionally non-pickable, so hover behavior is unchanged. Nonlinear and reversed axes, offset/scale metadata, continuous and categorical color, colorbars, and retained-payload GL reconstruction are covered.

Performance evidence

Fresh Chromium/SwiftShader benchmark, grid size 256, 66,049 occupied cells, 9 repetitions:

metric	untouched main	this PR
resident vertex-attribute bytes	11,096,232	792,588
build/upload median	3.2 ms	0.1 ms
build/upload p95	5.2 ms	0.3 ms
draw median	0.3 ms	0.2 ms
draw p95	0.4 ms	0.4 ms

That is an exact 14× / 92.86% resident-buffer reduction. Both implementations rendered 4,528 nonblank pixels with pixel hash 3904178501 and GL error 0.

An independent review reran the same comparison at 16,641 occupied cells:

• 2,795,688 → 199,692 bytes (also exact 14×)
• build/upload median/p95: 0.8/1.1 → 0.1/0.2 ms
• both paths: 4,618 nonblank pixels, hash 3001618805, GL error 0

Verification

• focused hexbin/browser/plot/colorbar coverage: 5 passed
• real Chromium cases include linear axes and log/reversed axes, exact buffer sizing, the 18-vertex instanced draw, pixel parity before/after zoom and retained-payload GL rebuild, old-buffer deletion, shader replacement, and no GL errors
• full suite: 2210 passed, 66 skipped, 1 deselected in 75.43 s; the sole deselection is the known pre-existing tests/reflex_adapter/test_assets.py::test_client_source_is_the_installed_bundle stale literal-marker assertion, reproduced on untouched main
• node js/build.mjs --check: committed bundles fresh
• tsc -p js/tsconfig.json --noEmit: passed
• ruff check .: passed
• ruff format --check .: all 343 files formatted
• git diff --check: passed

A reproducible benchmark is committed as benchmarks/bench_hexbin_client.py.

[codspeed-hq]

Merging this PR will not alter performance

✅ 97 untouched benchmarks
⏩ 1 skipped benchmark¹

---

_{Comparing agent/instanced-hexbin (15c9f10) with main (2a7d898)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[greptile-apps]

Greptile Summary

This PR moves hexbin fan generation from CPU-expanded buffers into an instanced WebGL renderer. The main changes are:

• Adds a vertex shader that generates each cell’s six triangles from gl_VertexID.
• Uploads one encoded center and color scalar per occupied cell.
• Updates draw dispatch, GPU reconstruction, cleanup, and generated browser bundles.
• Adds browser tests, a focused benchmark, and renderer contract documentation.

Confidence Score: 5/5

This looks safe to merge.

No blocking issues found in the changed code.

T-Rex Logs

What T-Rex did

• Ran the hexbin client instancing test with Playwright, and the run completed with 2 passed in 3.23s.
• Compared the poster frames before and after the change; the frames are pixel-identical, and the after recording exercises retained-payload GL teardown and rebuild.
• Reviewed the full artifact set (logs, videos, and images) to support validating the test outcomes and GL teardown behavior.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
js/src/40_gl.ts	Adds the instanced hexbin vertex shader and reuses the mesh fragment shader for color, opacity, and edge coverage.
js/src/50_chartview.ts	Replaces expanded triangle attributes with compact center buffers and adds dedicated hexbin program, VAO, and draw handling.
js/src/55_marks.ts	Routes hexbin marks through the new instanced draw path while retaining axis and theme refresh behavior.
tests/test_hexbin_client_instancing.py	Covers compact buffer sizing, instanced drawing, nonlinear axes, zoom, resource teardown, and retained-payload reconstruction.
benchmarks/bench_hexbin_client.py	Adds a browser benchmark for upload time, synchronized drawing, resident buffer size, and rendered output.
python/xy/static/index.js	Updates the shipped ESM bundle with the instanced hexbin renderer.
python/xy/static/standalone.js	Updates the shipped standalone bundle with the instanced hexbin renderer.

_{Reviews (1): Last reviewed commit: "Use instancing for compact hexbin render..." | Re-trigger Greptile}