table: Fit a column to its content on double-click of its resize handle

[miimsam]

Description

Double-clicking a column's resize handle now fits the column to its content, as spreadsheets and macOS table views do. The table can't measure what a delegate draws, so it asks the new TableDelegate::fit_column hook for the content width. The table then adds the cell padding, limits the result to the column's min_width and max_width, and emits TableEvent::ColumnWidthsChanged when the width changes. The hook returns None by default, so existing tables behave as before.

The fitted width is rounded up to whole device pixels. Layout rounds widths to the nearest device pixel, and text even a fraction of a pixel wider than its cell gets truncated. Each part (content and both paddings) is rounded separately, and at fractional scale factors the content is bumped one more device pixel when dividing back lands just below it.

The second press of a double-click also starts a drag on the handle, which would snap the fitted edge back under the pointer. Resize drags on a fitted column are ignored until the next single press.

The Data Table story implements the hook. One format_cell function now produces the text for both render_td and fit_column, so a fit measures exactly what is drawn. English and Chinese docs describe the hook, fit_col and col_width.

Before:

Screen.Recording.2026-10-13.at.12.18.45.mov

After:

Screen.Recording.2026-10-13.at.12.11.33.mov

Public API

gpui-component

• TableDelegate::fit_column(&mut self, col_ix: usize, window: &mut Window, cx: &mut Context<TableState<Self>>) -> Option<Pixels>: returns the width column col_ix's content needs (widest cell or header, as drawn), or None to leave the column unchanged. Called on a double-click of the column's resize handle; returns None by default.
• TableState::fit_col(&mut self, col_ix: usize, window: &mut Window, cx: &mut Context<Self>) -> bool: fits a column from code, the same as a double-click. Returns whether the delegate gave a width (false for a non-resizable column or a None answer).
• TableState::col_width(&self, col_ix: usize) -> Option<Pixels>: the width a column is drawn at now, following drags and fits; None past the last column.

How to Test

cargo test -p gpui-component --test integration -- table_fit_column::

Manually, in cargo run → Data Table:

• Double-click the divider after Symbol, Price, Change % and Name: each column fits its widest text, header included.
• Double-click the divider of a sortable column: the header text and sort icon both fit.
• Double-click and then drag slightly: the fitted width holds. A fresh single press and drag resizes as before.

Checklist

• I have read the CONTRIBUTING document and followed the guidelines.
• Reviewed the changes in this PR and confirmed AI generated code (If any) is accurate.
• Passed cargo run for story tests related to the changes.
• Tested macOS, Windows and Linux platforms performance (if the change is platform-specific) -> Tested only on macOS

AI-assisted: implementation, tests and documentation drafted with Claude Code, then reviewed and revised by me.

[huacnlee]

Thank you, we need this feature very much.