Underline a cell

What it does

Select cells and press Ctrl+U — or click the Underline button in the toolbar, or Format > Text > Underline in the menu bar — to toggle underline. Underline is presentation, separate from the cell's value: underlining =A1+B1 draws a line under the number without touching the formula. It undoes and redoes like any other edit, and survives a later value edit.

Verified behavior

Matches Google Sheets: In Google Sheets, Ctrl+U toggles underline on the selected cells (Cmd+U on Mac). Underline is a presentation attribute independent of the cell's value or formula, and it undoes/redoes like any edit. Toggling again removes it. Format > Text in the Sheets menu bar lists Bold, Italic, Underline, Strikethrough in that order — this codebase shipped Bold/Italic/Strikethrough but never Underline; a live audit against the reference workbook found the gap and this feature closes it.

Ctrl+U underlines the selected cell

Given

• A1 = hello

When I click A1, then press Control+U

Then

• A1 is underline

Ctrl+U a second time removes the underline

Given

• A1 = hello

When I click A1, then press Control+U, then press Control+U

Then

• A1 is underline=false

Format > Text > Underline toggles the same attribute as the keyboard chord

Given

• A1 = hello

When I click A1, then click menu item format.underline

Then

• A1 is underline

Underline survives undo/redo like any other edit

Given

• A1 = hello

When I click A1, then press Control+U, then press Control+Z

Then

• A1 is underline=false

Underline survives a value edit (presentation stays separate from content)

Given

• A1 = hello

When I click A1, then press Control+U, then type world into A1

Then

• A1 shows world
• A1 is underline