feat(sheets): add keyboard shortcut parity with other spreadsheets

[the-narwhal]

Keyboard shortcut parity with Excel / Google Sheets

Note: The @rowsncolumns/spreadsheet and @rowsncolumns/grid vendor packages live in a vendor/ subtree that is absent from this checkout, so this could not be built or run locally. The implementation is based on a thorough reading of the surrounding application code (ui-state.ts, state.ts, ui-store.tsx, LegacyGrid.tsx) and is intended as a reference or starting point for Proton to review, test, and iterate on internally.

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Problem

Several common Shift-based selection shortcuts were missing from the spreadsheet editor. The most reported example was Shift+Home having no effect, when users expect it to extend the selection to column A — consistent with both Excel and Google Sheets behaviour. Because the keyboard handling is entirely internal to the CanvasGrid from @rowsncolumns/spreadsheet, missing shortcuts cannot be added by configuring the component — they have to be intercepted externally.

Approach

A new hook, useGridKeyboardShortcuts, is registered in LegacyGrid and attaches a single keydown listener to document in the capture phase. This ensures our handler fires before the library's bubble-phase listeners. stopPropagation() is called only for the specific shortcuts this hook owns — every other keystroke passes through to the library completely unchanged.

Two guards prevent the hook from acting in the wrong context:

1. document.activeElement must be inside the grid's container element (no effect when a menu, toolbar, or dialog has focus)
2. The focused element must not be an <input>, <textarea>, or contenteditable — i.e. the user is not mid-edit inside a cell or the formula bar

All mutable values (activeCell, activeSheetId, rowCount, columnCount, setter functions) are read through refs that are refreshed every render, so the effect's empty dependency array never captures stale state.

Shortcuts added

Keys	Behaviour
Shift + Home	Extend selection to column A of the current row
Ctrl/⌘ + Shift + Home	Extend selection to A1
Shift + End	Extend selection to the last column of the current row
Shift + Space	Select the entire row
Ctrl + Space	Select the entire column
Ctrl + Shift + Space	Select all cells (anchor moves to A1)

Shift+End intentionally selects to the last allocated column (sheet edge), which matches Google Sheets behaviour — it is distinct from Ctrl+Shift+End, which is supposed to find the last cell with actual data.

Not yet implemented — and why

Ctrl+End / Ctrl+Shift+End — these are meant to navigate/extend to the last cell with data, not the last row/column of the allocated sheet. Using rowCount/columnCount directly would land on row 1000, column Z in an empty sheet, which would be actively wrong. Implementing these correctly requires knowing the signature of getDataRowCount (or an equivalent) from the vendor package.

Home / Ctrl+Home (navigation without Shift) — the library almost certainly handles these already. Overriding them in the capture phase without first confirming they are broken risks silently replacing library-provided scroll animation and visual feedback (cell flash, etc.). Worth verifying against the vendor source before adding.

Files changed

• Spreadsheet/useGridKeyboardShortcuts.ts (new) — the hook; self-contained with inline documentation of every shortcut, the ref pattern, and the two open TODOs
• Spreadsheet/components/legacy/LegacyGrid.tsx — one line added: useGridKeyboardShortcuts() at the top of `LegacyGrid