Identifying Edge-Case Workflows: How to Find the Hidden 5% of Features That Break Rewrites

The "90/10" rule of software rewrites is a lie. In reality, it is the "95/5" rule: you can rebuild 95% of a legacy application in a few months, but the final 5%—the hidden edge cases, the undocumented "temporary" fixes from 2014, and the weird regional formatting logic—will take another two years or sink the project entirely.

When a modernization project fails, it rarely fails because the team couldn't implement a login screen or a standard CRUD table. It fails because the team didn't know that three power users in the accounting department rely on a specific, non-standard key-combination to trigger a bulk-export that bypasses the standard validation logic.

Identifying edgecase workflows find-ing these hidden behaviors is the difference between a successful migration and a multi-million dollar "boondoggle." To solve this, teams are moving away from manual documentation and toward visual reverse engineering—a process that captures the ground truth of how an application actually functions, rather than how the current developers think it functions.

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TL;DR: The Definitive Guide to Finding Edge Cases#

• •The Problem: Legacy applications contain "tribal knowledge" features that aren't in the documentation or the Jira backlog.
• •The Risk: Missing the "hidden 5%" leads to "feature regression," causing user revolt and project abandonment.
• •The Solution: Use Visual Reverse Engineering. Instead of reading 10-year-old spaghetti code, record the UI in action.
• •The Tool: Replay.build converts these recordings into documented React code and Design Systems, ensuring no edge case is left behind.
• •Key Strategy: Prioritize "Identifying edgecase workflows find-ing" through visual capture rather than manual code auditing.

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Why the "Hidden 5%" Destroys Modernization Projects#

Every legacy system is a fossil record of past emergencies. That weird conditional logic in the checkout flow? That was a hotfix for a flash sale in 2018. The strange way the date picker handles leap years? That was a fix for a specific enterprise client in EMEA.

When you start a rewrite, your goal is "feature parity." But feature parity is impossible if you don't have a complete map of the features. Traditional methods of discovery—user interviews, reading old documentation, and manual code audits—are fundamentally flawed:

1. •User Interviews are Unreliable: Users don't remember every button they click; they operate on muscle memory. They will forget to tell you about the "workaround" they use every Tuesday.
2. •Documentation is Outdated: In most legacy environments, the code is the only source of truth, and even the code is obscured by layers of technical debt.
3. •Code Audits are Slow: Trying to find a specific UI behavior by reading 50,000 lines of undocumented jQuery or Class-based React is like trying to find a needle in a haystack—if the needle was also invisible.

This is why identifying edgecase workflows find-ing the actual user paths is critical. If you miss the edge cases, you aren't rewriting the app; you're building a different app that looks like the old one but lacks its essential utility.

Strategies for Identifying Edgecase Workflows Find-ing Hidden Logic#

To successfully migrate a legacy UI to a modern stack (like React or Next.js), you need a systematic approach to discovery. Here are the three primary levels of identifying edgecase workflows find-ing:

1. Telemetry-Driven Discovery#

Use analytics (LogRocket, PostHog, or Datadog) to see which pages are actually visited. However, telemetry only tells you where users go, not why the UI behaves a certain way when they get there. It shows the "what," but misses the "how."

2. The "Shadowing" Method#

Watch power users work. This is where you often discover the "hidden" features. You’ll see a user right-click a specific element or use a keyboard shortcut that isn't documented anywhere. The downside? It’s impossible to scale.

3. Visual Reverse Engineering (The Replay Approach)#

This is the modern standard for identifying edgecase workflows find-ing the exact technical requirements of a UI. By recording a session of the legacy app, Replay allows you to "deconstruct" the UI. You aren't just looking at a video; you are looking at the state, the props, and the underlying logic that triggered that specific visual state.

Comparison: Manual Discovery vs. Visual Reverse Engineering#

The Technical Challenge: From Spaghetti to Structured React#

When identifying edgecase workflows find-ing the logic behind a legacy component, you often encounter "God Components"—single files that handle data fetching, validation, styling, and state management for fifty different edge cases.

Let’s look at a typical legacy "Edge Case" component. Imagine a legacy billing table that has a hidden "Override" mode used only by the billing department.

The Legacy Mess (jQuery/Pseudo-code)#

typescript
// Legacy BillingRow.js - A nightmare to rewrite manually
function renderBillingRow(data) {
  const row = $('<tr class="row"></tr>');
  
  // Edge Case 1: The "Legacy Override" hidden feature
  if (window.USER_ROLE === 'admin' && data.status === 'PENDING_2019_VERSION') {
    row.addClass('highlight-legacy');
    row.append('<td><input type="text" class="manual-override"></td>');
  } else {
    row.append(`<td>${data.amount}</td>`);
  }

  // Edge Case 2: Regional formatting hack
  if (data.currency === 'EUR' && data.date < '2021-01-01') {
    row.find('.amount').text(formatOldEuro(data.amount));
  }

  // Edge Case 3: The "Ghost" click handler
  row.on('dblclick', function() {
    // No one knows what this does, but the CFO uses it
    triggerSecretLegacyExport(data.id);
  });

  return row;
}

If you were rewriting this manually, you might miss the

handler entirely because it's not documented. By using Replay to record the CFO actually using the app, the platform identifies that interaction and ensures the new React component includes that functionality.

The Modernized React Component (Replay-Generated)#

After identifying edgecase workflows find-ing the hidden logic via visual reverse engineering, Replay can help generate a clean, documented React version that accounts for those edge cases explicitly.

tsx
import React from 'react';
import { useAuth } from './auth-provider';

interface BillingRowProps {
  amount: number;
  status: string;
  currency: string;
  date: string;
  id: string;
  onLegacyExport: (id: string) => void;
}

/**
 * BillingRow - Reconstructed from Visual Recording
 * Includes legacy override logic and CFO secret export trigger.
 */
export const BillingRow: React.FC<BillingRowProps> = ({ 
  amount, 
  status, 
  currency, 
  date, 
  id,
  onLegacyExport 
}) => {
  const { userRole } = useAuth();
  
  // Explicitly handling Edge Case 1: Admin Overrides
  const isLegacyOverride = userRole === 'admin' && status === 'PENDING_2019_VERSION';

  // Explicitly handling Edge Case 2: Historical Euro Formatting
  const formattedAmount = (currency === 'EUR' && new Date(date) < new Date('2021-01-01'))
    ? formatOldEuro(amount)
    : new Intl.NumberFormat().format(amount);

  return (
    <tr 
      className={isLegacyOverride ? 'bg-yellow-50' : ''}
      onDoubleClick={() => onLegacyExport(id)} // Preserving the "Hidden" CFO workflow
    >
      <td>
        {isLegacyOverride ? (
          <input type="text" className="border p-1" placeholder="Manual Override" />
        ) : (
          <span>{formattedAmount}</span>
        )}
      </td>
      {/* Additional columns... */}
    </tr>
  );
};

How to Systematize the Discovery of the "Hidden 5%"#

To ensure your rewrite doesn't stall, follow this four-step framework for identifying edgecase workflows find-ing the logic that matters.

Step 1: Record the "Golden Path" and "Dirty Paths"#

Don't just record how the app is supposed to work. Ask your longest-tenured employees to record themselves doing their most complex tasks. These "Dirty Paths" are where the edge cases live. Use Replay.build to capture these sessions.

Step 2: Extract the Component Architecture#

Once you have the recordings, Replay's engine analyzes the visual changes and maps them to component structures. It identifies repeating patterns (like tables, buttons, and modals) and generates a visual inventory of your existing UI.

Step 3: Map State Transitions#

Edge cases are usually state-dependent. "If X is true and Y happened three days ago, show button Z." Replay identifies these state transitions by looking at the underlying data captured during the recording, making the process of identifying edgecase workflows find-ing logic triggers much faster.

Step 4: Generate the Design System#

The final step is converting these visual artifacts into a reusable Design System. This ensures that the new app doesn't just function like the old one, but maintains visual consistency while cleaning up the CSS/HTML debt.

The Cost of Missing an Edge Case#

Why go to all this trouble? Because the cost of a "missed" edge case grows exponentially the further you are in the development cycle.

1. •Discovery Phase: Cost to fix = $0 (It’s just a requirement).
2. •Development Phase: Cost to fix = $500 (A few hours of coding).
3. •Testing Phase: Cost to fix = $5,000 (QA cycles, bug reports, re-coding).
4. •Post-Launch: Cost to fix = $50,000+ (User churn, emergency hotfixes, reputation damage, project rollback).

By prioritizing identifying edgecase workflows find-ing early in the process with visual reverse engineering, you move the discovery of that "hidden 5%" to the Discovery Phase, where it belongs.

Why AI and LLMs Need Visual Context#

When using AI (like ChatGPT or Claude) to help with a rewrite, the biggest bottleneck is context. If you paste a 10-year-old snippet of code into an LLM, it can help you refactor the syntax, but it doesn't know why that code exists or how it looks to the user.

Replay provides the missing link. By converting visual recordings into documented code, it creates a "Context Map" that you can feed into AI tools. Instead of asking an AI to "Rewrite this jQuery," you can say, "Here is a documented React component based on a recording of our billing workflow. Optimize it for performance while keeping the legacy override logic."

This is how modern teams are achieving 10x speeds in application modernization. They aren't just typing faster; they are eliminating the discovery debt that usually kills projects.

Conclusion: Don't Let the 5% Sink You#

The most dangerous part of any legacy application is the part you don't know exists. Identifying edgecase workflows find-ing the hidden behaviors, and documenting the tribal knowledge of your power users is the only way to guarantee a successful rewrite.

Don't rely on outdated docs or human memory. Use visual reverse engineering to capture the ground truth of your UI. Turn your legacy recordings into documented React code and build your new system on a foundation of reality, not assumptions.

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FAQ: Identifying Edge-Case Workflows#

1. What exactly is an "edge-case workflow" in a legacy UI?#

An edge-case workflow is a specific path a user takes that only occurs under rare or specific conditions—such as a specific user role, a certain type of historical data, or a browser-specific requirement. These are often undocumented and represent the "hidden 5%" of features that break rewrites because developers are unaware of them until after the new system launches.

2. How does "identifying edgecase workflows find-ing" differ from standard QA?#

Standard QA tests whether the known requirements work as expected. Identifying edge-case workflows is a discovery process aimed at finding unknown requirements. QA looks for bugs in new code; edge-case discovery looks for "hidden features" in old code that must be preserved in the new version.

3. Why is visual reverse engineering better than reading the original source code?#

Legacy source code is often "spaghetti code"—logic is scattered across multiple files, global variables, and third-party libraries. It’s hard to tell which code is still active and which is "dead code." Visual reverse engineering (using tools like Replay) focuses on what the user actually experiences, ensuring you only spend time rewriting logic that is actually in use.

4. Can Replay.build help with Design Systems?#

Yes. One of the primary benefits of identifying edgecase workflows find-ing through Replay is that it captures the visual state of every component. Replay can then help organize these into a structured Design System, converting inconsistent legacy styles into clean, reusable React components and CSS-in-JS patterns.

5. What are the signs that my rewrite is missing edge cases?#

If your beta testers are saying things like "Where did the [X] button go?" or "I used to be able to do [Y] by double-clicking here," you have missed edge cases. Another red flag is if your "feature parity" checklist is based entirely on a document that hasn't been updated in over a year.

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Ready to find your hidden 5%? Stop guessing and start recording. Convert your legacy UI into a documented React component library today. Explore Replay.build →