The Width: 100% Mystery - Debugging a Deceptively Simple CSS Centering Bug

Sometimes the hardest bugs to fix are the ones that look the simplest. Yesterday, I spent way too long debugging what appeared to be a straightforward CSS centering issue on mobile. The blue navigation buttons (Blog▼, Projects▼) stayed perfectly centered, but the white dropdown items kept shifting left and aligning with the screen edges.

This is the story of how I finally solved it—and more importantly, why it was so hard to spot in the first place.

The Problem: A Tale of Two Behaviors

Here's what I was seeing on mobile:

• Top-level nav buttons (Home, Blog▼, Projects▼, About): Perfectly centered ✅
• Dropdown items (All Blog Posts, Technology & Development, etc.): Shifted left, hugging screen edges ❌

The strange part? Both sets of elements were in the same navigation system, styled with similar CSS, and had all the "right" centering properties. Or so I thought.

The Investigation: Why Nothing Worked

As any developer would, I started throwing common centering solutions at the problem:

Attempt #1: Text Alignment

.dropdown-item {
  text-align: center !important;
}

Result: The text centered within its container, but the container itself still hugged the left edge. ❌

Attempt #2: Flexbox Centering

.dropdown-item-content {
  align-items: center !important;
  justify-content: center !important;
}

Result: Same problem. The content was centered inside, but the boxes were still wrong. ❌

Attempt #3: Width Constraints

.dropdown-item {
  width: fit-content !important;
  max-width: calc(100% - 2rem) !important;
}

Result: Better! But still not centered like the top-level buttons. ❌

At this point, I was frustrated. Everything looked correct. The CSS was there. The browser DevTools showed my styles applied. Why weren't the dropdown items centering like the top-level nav buttons?

The Breakthrough: Question Your Assumptions

The solution came when I stopped looking at the dropdown items themselves and started looking at their parent containers.

Here's what I found buried in the mobile styles:

/* Line 565 in Navbar.js */
.navbar-links.open .nav-dropdown {
  align-self: center;
  text-align: center;
  width: 100%;  /* 👈 THE CULPRIT */
}

/* Line 579 */
.dropdown-menu {
  width: 100%;  /* 👈 ALSO PROBLEMATIC */
}

There it was. The dropdown containers had width: 100%, which meant they were stretching edge-to-edge across the screen. The dropdown items inside them had no choice but to follow along.

Meanwhile, the top-level nav buttons? They had no width constraint at all. They sized naturally to their content and centered beautifully.

The Solution: Let Go of Control

The fix was almost embarrassingly simple:

/* Remove the width constraint */
.navbar-links.open .nav-dropdown {
  align-self: center;
  text-align: center;
  /* width: 100%; ❌ REMOVED */
}

/* Change to natural sizing */
.dropdown-menu {
  width: auto;  /* 👈 Changed from 100% */
  max-width: calc(100vw - 2rem);  /* Prevent overflow */
  margin: 0 auto;  /* Center the container */
}

By removing the width: 100% constraint and letting the dropdown containers size naturally (width: auto), they could now center just like the top-level buttons. The margin: 0 auto ensured they stayed centered horizontally.

The Junior Dev Explanation: A Simple Analogy

Imagine you're trying to center a picture frame on your wall:

• Scenario 1 (Top-level buttons): You have a small picture frame. You measure the wall, find the center, and hang it there. Easy! ✅

• Scenario 2 (Dropdown items with width: 100%): Someone gives you a picture frame that's as wide as the entire wall. Now when you try to "center" it, it just spans the whole wall edge-to-edge. You can center the picture inside the frame all you want, but the frame itself can't be centered because it's already touching both edges. ❌

That's exactly what was happening with width: 100%. The container was taking up the full width, so centering had nowhere to go.

Why This Bug Was So Hard to Spot

Looking back, there were several factors that made this bug surprisingly difficult to debug:

1. The Styles Were in Different Places

The navigation component used styled-jsx with media queries embedded in the component file. The problematic width: 100% was on line 565, while the dropdown styles I kept modifying were on line 598. My eyes kept focusing on the wrong section.

2. Mental Model Mismatch

I was thinking: "The items aren't centered, so I need to add centering properties."

I should have been thinking: "The items aren't centered because something is preventing them from centering. What constraint needs to be removed?"

3. The Top-Level Buttons Worked Perfectly

Since the blue buttons worked great, I assumed the mobile navigation system was fundamentally sound. I didn't think to compare how the two elements differed at the container level.

4. The text-align: center Red Herring

Adding text-align: center did something—it centered the text inside the boxes. This made me think I was on the right track, when actually I was solving the wrong problem.

Key Takeaways for Future Debugging

Here's what I learned from this experience:

1. Start with Constraints, Not Properties

When an element won't center, check for constraints that prevent centering:

• width: 100% on the element or its parents
• position: absolute without proper left/right values
• Flexbox parents without proper justify-content
• Grid parents without proper alignment

2. Compare What Works vs. What Doesn't

The top-level buttons worked perfectly. I should have immediately compared their CSS with the dropdown items' CSS, focusing on differences rather than similarities.

3. Use Browser DevTools Effectively

Chrome DevTools shows the computed box model. I should have used it to visualize why the dropdown containers were spanning the full width.

4. Question "Obvious" Solutions

When text-align: center and align-items: center don't work, that's a signal to look upstream at container constraints, not downstream at more specific properties.

5. Read the Full Context

In complex components with media queries and multiple nested styles, it's easy to focus on one section and miss the bigger picture. Sometimes you need to step back and read the entire stylesheet with fresh eyes.

The HTML Structure (For Context)

Here's what the HTML structure looked like:

<div className="navbar-links open">
  <div className="nav-dropdown"> {/* 👈 Had width: 100% */}
    <Link className="nav-link">Blog ▼</Link>
    <div className="dropdown-menu"> {/* 👈 Also had width: 100% */}
      <Link className="dropdown-item">
        <div className="dropdown-item-content">
          <span className="dropdown-item-title">All Blog Posts</span>
        </div>
      </Link>
    </div>
  </div>
</div>

The width: 100% on both .nav-dropdown and .dropdown-menu meant the containers filled the entire width of .navbar-links, leaving no room for centering.

A Better Debugging Checklist

Next time I encounter a centering issue, here's the mental checklist I'll use:

1. ✅ Is the element constrained by its parent's width? (width: 100%, fixed widths)
2. ✅ Is the parent a flex or grid container with the right alignment properties?
3. ✅ Are there conflicting positioning values? (position, left, right)
4. ✅ Is the element's display property compatible with centering? (inline, block, etc.)
5. ✅ Are there inherited styles from global CSS or parent components?

Only after checking these should I start adding new centering properties.

Conclusion: Simple Problems, Deep Lessons

This bug looked trivial on the surface: "Just center these dropdown items." But it taught me several valuable lessons about CSS debugging, mental models, and the importance of questioning assumptions.

The next time you're stuck on what seems like a "simple" CSS problem, remember:

• Check parent constraints first
• Compare what works vs. what doesn't
• Question whether you need to add properties or remove constraints
• Use DevTools to visualize the box model
• Step back and read the full context

And most importantly: sometimes the best solution isn't adding more code—it's removing the code that's getting in the way.

Got a CSS debugging story of your own? I'd love to hear it! Feel free to share your experiences in the comments below.