Error cells show a diagnostic tooltip

What it does

When a cell resolves to an error — #REF!, #DIV/0!, #NAME?, #VALUE!, #NUM!, #N/A — hovering it or making it the active selection shows a small red-accented tooltip: an "Error" header plus a plain-language diagnostic line (e.g. "Division by zero." for #DIV/0!). It tracks the cell and disappears when you move the pointer away, select a different cell, start editing, or scroll.

Try it

Verified behavior

Matches Google Sheets: In Google Sheets, a cell that resolves to an error (#REF!, #DIV/0!, #NAME?, #VALUE!, #NUM!, #N/A, …) shows a small red-accented tooltip with an "Error" header and a plain-language diagnostic line — both when you hover the cell and when it's the active selection. The tooltip tracks the cell it's anchored to and disappears when you move the mouse away or scroll; a non-error cell never shows one. TrueCalc's engine (@truecalc/workbook) does not yet emit a rich per-error diagnostic message (core#731, itself blocked upstream on the Google Sheets conformance-fixtures pipeline capturing real error-message strings, which hasn't landed yet) — so this feature ships with a FALLBACK per-code message table (the six codes the engine currently surfaces) rather than the engine's own wording, and is structured to prefer an engine-supplied message the moment one exists (see domain/error-message.ts).

A

Given

• A1 = =1/0

When I click A1

Then

• A1 shows #DIV/0!
• [data-slot="error-tooltip"] is visible containing "Division by zero."

A non-error cell shows no tooltip when selected

Given

• A1 = 42

When I click A1

Then

• A1 shows 42
• [data-slot="error-tooltip"] is hidden