Fix heatmap hover pick raising IndexError in the kernel handler

[FarhanAliRaza]

Summary

Hovering (or clicking) a heatmap in a notebook crashed the Python-side pick handler with an IndexError on every mouse move, spamming the cell output with tracebacks via ipywidgets' CallbackDispatcher. The chart kept rendering — only the hover readout died.

Root cause

pick() (python/xy/interaction.py) assumed the point-like invariant len(t.x.values) == t.n_points. Heatmap traces violate it: they store only the two outer grid edges as x/y while n_points is rows * cols, so the bounds guard passed and t.x.values[cell_index] indexed a 2-element array. The resulting IndexError isn't caught by channel.py's except (TypeError, ValueError), so it escaped the comm handler.

Fix

Grid traces (t.grid_shape is not None) now take a dedicated branch in pick() that returns the bin readout per dossier §17: trace, index, row, col, and color_value — the exact f64 z from the canonical grid (honoring §16's f64-hover rule; omitted for NaN cells). Out-of-range indices return None. The same branch fixes the identical crash in the click path.

Two deliberate choices:

• x/y are omitted from the row. The client merge lets kernel fields override the local row, and heatmap axes can be categorical (labels live figure-side while trace columns are numeric) — a kernel numeric x would replace category labels with raw numbers in the tooltip. The client's local row already shows the correct cell center; the kernel's genuine contribution is the exact z value. A test pins the omission.
• Not suppressed with row: None. The client hides the tooltip on a null pick_result row, which would have erased the working local readout.

Testing

• New: test_pick_heatmap_cell_returns_grid_readout, test_pick_heatmap_out_of_range_returns_none, test_pick_heatmap_nan_cell_omits_value (tests/test_scatter.py) and test_heatmap_pick_message_replies_without_raising (tests/test_channel.py, mirrors the heatmap+contour repro through handle_message).
• uv run pytest tests/test_scatter.py tests/test_channel.py -q — 97 passed
• uv run pytest tests/ -q -k "pick or heatmap" — 57 passed, 1 skipped
• pre-commit run --all-files, ruff check, ruff format --check — clean

Closes #89

[codspeed-hq]

Merging this PR will not alter performance

✅ 94 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing fix/heatmap-pick-indexerror (3158eb6) with main (d3e9dfe)}

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. ↩