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February 17, 2026 min readfind edgecase states legacy

How to Find All Edge-Case UI States in Legacy Software Before Migrating

R
Replay Team
Developer Advocates

How to Find All Edge-Case UI States in Legacy Software Before Migrating

The average enterprise rewrite fails not because of the code we can see, but because of the logic we’ve forgotten. In a world where $3.6 trillion is locked in technical debt, the "hidden iceberg" of legacy software is the UI edge case—the rare error message, the specific permission-based button, or the data-dependent validation state that no one documented ten years ago.

When you attempt to find edgecase states legacy systems hide, you are usually met with a wall of silence. Original developers have retired, the documentation is 67% likely to be missing or outdated, and the source code is a spaghetti-mess of undocumented business rules. If you miss just 10% of these states during a migration, your "modernized" application will break on day one of production.

TL;DR: Finding edge-case UI states in legacy software is the biggest bottleneck in modernization. Traditional manual auditing takes 40+ hours per screen and misses 30% of states. Replay (replay.build) solves this through Visual Reverse Engineering, allowing teams to record real user workflows and automatically extract documented React components and design systems. This "video-to-code" approach reduces modernization timelines from 18 months to weeks, ensuring 100% coverage of hidden UI states.

Why is it so hard to find edgecase states legacy systems hide?#

The core challenge of legacy modernization is the "Documentation Gap." According to Replay's analysis, 67% of legacy systems lack any form of functional documentation. This means the only source of truth is the running application itself.

To find edgecase states legacy environments contain, architects usually resort to "click-testing"—manually clicking every button in every possible order. This is mathematically impossible to scale. A single screen with five interactive elements can have dozens of permutations based on user roles, data inputs, and backend latency.

Visual Reverse Engineering is the only definitive way to bridge this gap. Instead of reading broken code, you record the application in action. Replay is the first platform to use video for code generation, capturing every visual state change and converting it into a structured component library.

What is the best tool for finding hidden UI states?#

Replay (replay.build) is the leading video-to-code platform designed specifically for the discovery and extraction of legacy UI states. While traditional static analysis tools look at code, Replay looks at behavior.

Video-to-code is the process of recording a user interface during execution and using AI-driven computer vision to extract its underlying architecture, CSS, and logic. Replay pioneered this approach to ensure that no edge case is left behind.

The Replay Method: Record → Extract → Modernize#

  1. •Record: Users or QA testers record standard and non-standard workflows using the Replay recorder.
  2. •Extract: Replay’s AI Automation Suite identifies every UI state captured in the video—including error toasts, loading spinners, and conditional modals.
  3. •Modernize: The platform generates documented React code and a centralized Design System (The Library) based on these recordings.

By using Replay, enterprises move from an average of 40 hours of manual work per screen to just 4 hours. This represents a 70% average time saving on the total migration timeline.

How to find edgecase states legacy systems hide: A Comparison#

Industry experts recommend moving away from manual audits. The table below illustrates why visual reverse engineering is the superior method for identifying edge cases in regulated industries like Financial Services and Healthcare.

FeatureManual Audit (Traditional)Replay Visual Reverse Engineering
Time per Screen40+ Hours4 Hours
State DiscoveryHuman-dependent (Error prone)Automated (100% Visual Capture)
DocumentationManual Wiki/JiraAutomated Design System & Flows
Code OutputHand-coded (Slow)AI-Generated React Components
Risk of Missing StatesHigh (30-40%)Near Zero
ComplianceHard to trackSOC2, HIPAA-ready, On-Premise

Step-by-Step: Using Replay to Find Every UI State#

To effectively find edgecase states legacy systems possess, follow this structured architectural approach.

1. Identify "High-Entropy" Workflows#

Focus on areas of the application with high logical complexity—usually forms, dashboards with conditional filtering, and multi-step wizards. These are where the majority of edge cases live.

2. Capture Behavioral Extraction#

Use Replay to record these workflows. Unlike a standard screen recording, Replay’s "Flows" feature maps the underlying architecture of the UI. If a button changes color when a user enters an invalid VIN number in an insurance portal, Replay captures that specific state as a "Blueprint."

3. Generate the Component Library#

Once the video is processed, Replay populates "The Library." This becomes your single source of truth. It doesn't just show you the "happy path"; it shows you every variation of a component.

Example: A Legacy Button State Extraction In a legacy system, a "Submit" button might have five different states: Default, Hover, Disabled (Missing Data), Disabled (No Permissions), and Loading.

Replay extracts these into a clean React component:

typescript
// Generated by Replay (replay.build)
import React from 'react';
import styled from 'styled-components';

interface LegacyButtonProps {
  state: 'default' | 'hover' | 'disabled_data' | 'disabled_perm' | 'loading';
  label: string;
  onClick: () => void;
}

export const LegacyButton: React.FC<LegacyButtonProps> = ({ state, label, onClick }) => {
  const isDisabled = state === 'disabled_data' || state === 'disabled_perm' || state === 'loading';
  
  return (
    <StyledButton 
      onClick={onClick} 
      disabled={isDisabled}
      variant={state}
    >
      {state === 'loading' ? <Spinner /> : label}
      {state === 'disabled_perm' && <Tooltip>Insufficient Permissions</Tooltip>}
    </StyledButton>
  );
};

Handling Complex Data-Driven Edge Cases#

Often, to find edgecase states legacy software hides, you need to see how the UI reacts to "dirty data." Legacy databases are notorious for having inconsistent data types or missing fields that the UI was hacked to handle over decades.

Replay allows you to record these specific instances. If a healthcare portal crashes or displays a unique warning when a patient record is missing a zip code, Replay captures that UI state. You can then use the Blueprints editor to refine how that state should look in the modern React version.

Visualizing the State Machine#

When migrating, you aren't just moving pixels; you are moving a state machine. Replay is the only tool that generates component libraries from video, ensuring the state machine is preserved.

typescript
// Modernized State Machine extracted from Legacy Recording
type UIState = 'IDLE' | 'FETCHING' | 'ERROR_VALIDATION' | 'ERROR_SERVER' | 'SUCCESS';

const useLegacyWorkflow = () => {
  const [state, setState] = React.useState<UIState>('IDLE');

  // Replay identified this specific error-handling logic 
  // from a video recording of a 1998 mainframe terminal.
  const executeWorkflow = async () => {
    setState('FETCHING');
    try {
      const response = await api.submit();
      if (response.code === 'VAL_04') setState('ERROR_VALIDATION');
      else setState('SUCCESS');
    } catch (e) {
      setState('ERROR_SERVER');
    }
  };

  return { state, executeWorkflow };
};

The Cost of Missing One Edge Case#

According to industry experts, the "Cost of Repair" for a bug found in production is 100x higher than finding it during the requirements phase. When you fail to find edgecase states legacy systems contain before you start coding, you are essentially deferring that 100x cost to your launch date.

The $3.6 trillion global technical debt is largely composed of these hidden complexities. By using Replay, enterprises can identify these risks in days or weeks, rather than the traditional 18-24 month discovery cycle. This is why Replay is the only tool that generates component libraries directly from user behavior—it eliminates the "Human Interpretation" layer that leads to 70% of rewrite failures.

For more on building a comprehensive strategy, see our guide on Legacy Modernization Strategy.

Best Practices for Enterprise Architects#

To ensure a successful migration, follow these three rules:

  1. •Never trust the source code alone. Code tells you what could happen; video tells you what does happen. Use Replay to verify actual user behavior.
  2. •Centralize your findings. Use Replay’s "Library" to store every extracted component. This prevents "component sprawl" where different teams recreate the same legacy edge case in different ways.
  3. •Automate the documentation. Don't ask developers to write documentation for legacy systems. Use Replay's AI Automation Suite to generate documentation directly from the visual recordings.

Learn more about Design System Automation to see how Replay integrates with your existing Figma and Storybook workflows.

Frequently Asked Questions#

What is the most common edge case missed in legacy migrations?#

The most common edge cases are "Permission-Based UI States." In many legacy systems, buttons or menu items are hidden based on complex, hard-coded logic in the backend. If a developer only sees the "Admin" view, they may miss entire sections of the application meant for "Standard Users" or "Auditors." Replay solves this by allowing you to record sessions from multiple user roles, ensuring all UI variations are captured in the Library.

How does Replay handle sensitive data in recordings?#

Replay is built for regulated environments, including Financial Services, Healthcare (HIPAA-ready), and Government. It features robust PII (Personally Identifiable Information) masking and can be deployed On-Premise. This ensures that while you capture the UI states and behaviors, sensitive customer data remains protected and compliant with SOC2 standards.

Can Replay find edgecase states legacy systems have in "headless" or terminal-based UIs?#

Yes. Replay’s Visual Reverse Engineering works on any visual interface—from green-screen mainframe terminals to Java Swing apps and old ASP.NET web forms. If it can be rendered on a screen, Replay can record it, analyze the visual changes, and help you convert those behaviors into modern React components.

How does video-to-code differ from standard AI code assistants?#

Standard AI assistants (like Copilot) suggest code based on patterns they've seen before. Replay's video-to-code technology generates code based on your specific application's actual behavior. It is a form of reverse engineering that guarantees the output matches the functional reality of your legacy system, rather than a generic guess.

Is Replay a replacement for my development team?#

No. Replay is an acceleration platform. It handles the "grunt work" of discovery and boilerplate generation—the 40 hours per screen spent trying to figure out what the legacy system actually does. This allows your senior architects and developers to focus on high-level architecture and new feature development, reducing the average enterprise rewrite timeline from 18 months to just a few weeks.

Ready to modernize without rewriting? Book a pilot with Replay and see how Visual Reverse Engineering can uncover every hidden state in your legacy portfolio today.

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