Fix standalone density flicker on pan/zoom-out

[adhami3310]

Problem

In a kernel-less to_html density export, the density visibly flickered on the slightest drag — swapping between the full-data overview grid and the retained sample. Two distinct causes:

1. Color/brightness flip on pan. _requestSampleRebin re-binned the sample whenever the view wasn't exactly at the home extent, so a pure pan swapped the full-data overview grid for a sample-derived grid normalized against a much smaller maximum. The two normalize differently, so the density jumped.

2. Points vanishing on pan/zoom-out. The retained sample overlay was gated by _viewInside(s.win), so its individual points dropped out the instant the view left the sample's home window, while the density stayed — an inconsistent "density loses its grain" effect.

Fix

1. Gate the re-bin on view span, not position: pans and zoom-outs keep (and restore) the full-data overview; only a genuine zoom-in re-bins, where the sample actually adds resolution the overview lacks.

2. Add _viewOverlaps and draw the sample whenever it overlaps the view. Points are positioned in data space and the GPU clips off-screen ones, so the "density + points" look now travels with the view and only drops when panned entirely off the data.

Scoped strictly to the standalone (no-kernel) path; the kernel density_view path is unchanged.

Tests

Adds tests/test_density_pan_no_rebin.py — a headless-Chromium regression test asserting:

• a pan keeps the overview and spawns no re-bin worker,
• a zoom-in still routes to the sample re-bin path,
• the sample overlay persists through a zoom-out but drops when panned entirely off the data.

Render smoke + related suites pass; bundles regenerated via node js/build.mjs; ruff/format clean.

[codspeed-hq]

Merging this PR will not alter performance

✅ 94 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing fix/standalone-density-pan-rebin (02dbc0f) with main (914a327)}

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. ↩

[masenf]

@greptile review 🙏

[greptile-apps]

Greptile Summary

This PR prevents standalone density flicker during pan and zoom-out. The main changes are:

• Keeps the full-data overview unless both view spans are tighter than the home view.
• Draws retained sample points whenever their data window overlaps the current view.
• Regenerates both browser bundles with the updated behavior.
• Adds a Chromium test for pan, zoom-in, zoom-out, and off-data views.

Confidence Score: 5/5

This looks safe to merge.

No blocking issues found in the changed code.

T-Rex Logs

What T-Rex did

• The focused test completed with exit code 0 and reported 1 passed in 1.45s.
• Validated the post-pan probe results, confirming viewChanged=true, densityIsOverview=true, workerCreated=false, sampleWindowOverlapsView=true, densitySampleDrawCallsAfterPan=24, and no browser errors.
• Verified that the PNG and WebM artifacts display the actual styled canvas chart produced by chart.to_html(), not synthetic markup.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
js/src/50_chartview.js	Uses view-window overlap to keep retained density points visible while any part of their data window remains on screen.
js/src/54_kernel.js	Uses view spans to choose between the full-data overview and sample re-binning, and adds the overlap helper.
python/xy/static/index.js	Regenerates the main browser bundle with the density changes.
python/xy/static/standalone.js	Regenerates the standalone browser bundle with the density changes.
tests/test_density_pan_no_rebin.py	Adds browser coverage for overview restoration, zoom-in re-binning, sample persistence, and off-data suppression.

_{Reviews (1): Last reviewed commit: "Fix standalone density flicker on pan/zo..." | Re-trigger Greptile}