fix: fill the pane with the last column and auto-fit worker-backed column widths

[singularitti]

Why this change is needed

Opening a file whose columns don't fill the editor width shows a blank, headerless "phantom column" after the last real column. The easiest repro is a 3-column JSONL — e.g. a Codex rollout log with timestamp / type / payload — viewed on a normal-width editor.

This looks like a parsing bug or a malformed file, but it is neither. The grid parses exactly the right columns (verified: every line of the sample file is valid NDJSON with the same three keys). The blank region is simply unused horizontal space:

• .grid-header and .grid-row are full-width flex containers (.grid-sizer is min-width: 100%).
• Cells are flex-shrink: 0 with fixed pixel widths and pack to the left.
• When the columns are narrower than the pane, the leftover width still carries the header's background and each row's bottom border — so it reads as an empty, unlabeled column.

It is most visible for worker-backed formats (JSON/Parquet/Arrow/Excel): these never received the auto-width pass that CSV gets, so their columns fell back to narrow type-default widths and rarely filled the pane. A wide field like payload was crammed into 160px while a large gap sat to its right.

before                                            after
┌──┬───────────┬────────┬─────────┬───────────┐   ┌──┬───────────┬────────┬──────────────────────────┐
│# │ timestamp │ type   │ payload │           │   │# │ timestamp │ type   │ payload                  │
├──┼───────────┼────────┼─────────┤  ← empty, ├   ├──┼───────────┼────────┼──────────────────────────┤
│1 │ 2026-03…  │ sessi… │ {"id":… │  no header│   │1 │ 2026-03…  │ sessi… │ {"id":"019cc4cd-ae7e-7e… │
└──┴───────────┴────────┴─────────┴───────────┘   └──┴───────────┴────────┴──────────────────────────┘

How it was implemented

1. Stretch the last column to fill the pane — webview-ui/src/components/DataGrid.svelte

The last visible (non-frozen) column now flex-grows to absorb the leftover width, so the grid always spans the pane with no phantom column. Because cells keep flex-shrink: 0 and their pixel width acts as the flex-basis, the column only ever grows past its natural width — when columns overflow the pane there is nothing to absorb and horizontal scrolling behaves exactly as before. It is pure CSS, so it re-fits on window/pane resize for free and the header and body stay aligned automatically. A frozen last column is skipped because position: sticky and flex-grow don't combine cleanly.

2. Auto-fit widths for worker-backed formats — webview-ui/src/stores/grid.svelte.ts

JSON/Parquet/Arrow/Excel never populate _csvAllRows, so the CSV/raw-rows auto-width pass never ran for them. Column widths are now fitted once from the first chunk on worker READY, so columns size to their content like CSV already does. A flag (reset on each READY) guards the pass so filtering or sorting — which re-requests chunk 0 — doesn't re-fit.

What it fixes

• No more blank "phantom column" after the last column for narrow datasets (JSONL, CSV, Parquet, Arrow, Excel, …).
• Columns fill the editor width; any spare space goes to the last column.
• JSON/Parquet/Arrow/Excel columns now auto-fit their content instead of falling back to fixed type-default widths.

The fill behavior applies to every file type, so narrow CSVs that previously showed the same trailing gap now fill the pane as well.

Behavior notes

• Since the last column always fills available width, dragging its resize handle narrower won't visibly shrink it while there's leftover space (it refills); dragging wider works and triggers horizontal scroll.
• The overflow case (more/wider columns than the pane) is unchanged — natural widths plus horizontal scroll.

Testing

• npm run build (extension + webview) passes; no new warnings.
• Packaged a .vsix and confirmed both changes are in the shipped bundle (.col-fill { flex-grow: 1 } in main.css; the auto-width path in main.js).
• Confirmed the sample JSONL parses to exactly three columns, so the data was never the problem.

🤖 Generated with Claude Code

[singularitti]

I made this change because I want to see all columns span the full window width, without any "empty column" gaps if the columns' total width is shorter than the window width. See examples below:

Before

After

[sciro24]

Looks good, the grid now fills the available width cleanly and the worker-backed auto-fit makes the initial column sizing much better.

[singularitti]

Thank you!