Invoice Builder Demo Spec

Goal

Build a complete invoice builder demo that exercises an editable line-item table, quantity and unit-price entry, computed line and invoice totals, tax and discount handling, and evidence-friendly rendering.

User Experience

The app opens to an invoice with a header, a few line items, and a totals summary. The user edits descriptions, quantities, and prices, adds or removes lines, and sees totals recompute immediately. The layout should be readable, fast to understand, and deterministic under a seed.

Layout

• An invoice header with an invoice number, a bill-to field, and an issue label.
• A line-item table with columns for description, quantity, unit price, and line total.
• A totals panel showing subtotal, discount, tax, and grand total.
• A status strip showing the line count and the grand total.

Controls

• Arrow keys: move the active cell across the line-item table.
• Enter / F2: edit the active cell.
• Tab / Shift+Tab: move to the next / previous editable field.
• N: add a new line item and focus its description.
• Delete: remove the focused line item.
• Esc: cancel the current edit.

Core Behaviors

• Each line item has a description, a quantity, and a unit price.
• A line total equals quantity times unit price, recomputed on each edit.
• The subtotal is the sum of all line totals.
• A discount is applied as a percentage of the subtotal; tax is applied to the discounted subtotal.
• The grand total equals subtotal minus discount plus tax.
• Quantity and price must reject non-numeric or negative input and restore the prior value.
• All money values must display with a fixed two-decimal format.

Data Model

• An ordered list of line items, each with a stable id, description, quantity, and unit price.
• A discount rate, a tax rate, and derived subtotal, discount, tax, and grand-total values.
• An active cell reference and an edit buffer.

Visual Requirements

• Show the header, the line-item table, the totals panel, and the status strip.
• The active cell must be clearly outlined.
• Numeric columns must be right-aligned and money values formatted consistently.
• The header and totals panel must keep stable dimensions while the table scrolls.

App State

Track at minimum:

• Line items with ids and fields, plus discount and tax rates.
• Derived totals, active cell reference, edit buffer, and validation state.
• Scroll offset and random seed.

Determinism and Evidence

• Accept an optional seed for any generated sample lines.
• Evidence mode should inject a deterministic script that edits a quantity, edits a unit price, and adds one line item.
• Evidence outcome should include frame count, edit count, line count, subtotal, grand total, and close reason.
• Screenshot evidence should show the line-item table and the totals panel.

Acceptance Criteria

• A line total equals quantity times unit price.
• The subtotal sums all line totals.
• Discount and tax apply in the defined order to produce the grand total.
• Non-numeric or negative quantity or price is rejected and the prior value restored.
• Adding and removing lines recomputes all totals.
• Money values display with two decimals.
• Header and totals panel stay fixed while the table scrolls.
• Interactive mode remains open until explicitly closed.

Out of Scope

• Multiple invoices or templates.
• Currency conversion or per-line tax rates.
• PDF export or persistence.
• External assets or audio.