Enter mode vs edit mode

What it does

There are two ways to start editing a cell, and they treat the arrow keys differently — exactly like Google Sheets:

• Enter mode — type a character. It replaces the cell's contents, and pressing an arrow commits the value and moves the selection one cell in that direction (the fast data-entry flow: 42 → Down → 7 → Down …).
• Edit mode — press F2, press Enter on a selected cell, or double-click. The existing content opens with a caret, and arrows move the caret through the text (Home / End jump to the ends). Double-click drops the caret near where you clicked.

Editing a formula (point mode)

While editing a formula, an arrow key does one of two things depending on where the caret is:

• Steer a reference ("point mode") — only when the caret is at the end of the text, right after =, an operator (+ - * / ^ & < > =), ( or ,. This is how you build a formula by pointing: type =, arrow to a cell; type +, arrow to the next.
• Move the caret — everywhere else. Moving the caret through a completed formula such as =B2+B1+C1 never rewrites a reference into it.

This caret gate is the fix for a data-corruption bug: previously any arrow, at any caret position, rewrote a reference into a formula you were only trying to move the caret through (=B2+B1 → =B2+B1+D42+C1).

Try it

Verified behavior

Matches Google Sheets: In Google Sheets there are two ways an in-cell edit begins, and they behave differently. TYPING a character starts "enter" mode: the character replaces the cell's contents and pressing an arrow commits the value and moves the selection one cell in that direction. F2, Enter on a selected cell, or a DOUBLE-CLICK start "edit" mode: the existing text opens with a caret and arrows move the caret through it (Home/End jump to the ends). While editing a FORMULA, an arrow steers a cell reference ("point mode") ONLY when the caret sits at the end of the text right after "=", an operator, "(" or "," — i.e. the formula is expecting a reference next. Anywhere else the arrow moves the caret, so moving through an existing formula such as =B2+B1+C1 never rewrites a reference into it. Every edit is undoable.

Typing a value then pressing an arrow commits it (enter mode)

Given

When I click A1, then press 4, then press 2, then press ArrowDown

Then

• A1 shows 42

Editing a formula with arrows moves the caret and never corrupts it

Given

• A1 = =B2+B1+C1
• B1 = 10
• B2 = 20
• C1 = 3

When I click A1, then press F2, then press ArrowLeft, then press ArrowLeft, then press ArrowRight, then press Enter

Then

• A1 shows 33
• A1 has formula =B2+B1+C1

Arrowing from the start through a formula inserts no references

Given

• A1 = =B2+B1
• B1 = 10
• B2 = 20

When I click A1, then press F2, then press Home, then press ArrowRight, then press ArrowRight, then press ArrowRight, then press Enter

Then

• A1 shows 30
• A1 has formula =B2+B1

Point mode — typing = then arrowing inserts a reference

Given

• B1 = 10

When I click A1, then press =, then press ArrowRight, then press Enter

Then

• A1 shows 10
• A1 has formula =B1

An edit is undoable

Given

• A1 = 5

When I click A1, then press 9, then press Enter, then undo

Then

• A1 shows 5