Scrollable and centered modals in Bootsrap - Performance & Optimization

Performance: Scrollable and centered modals

MEDIUM IMPACT

This affects page interaction speed and visual stability when modals appear and scroll within the viewport.

Creating a modal that is scrollable and vertically centered on the screen

Bootsrap

<div class="modal fade" tabindex="-1" role="dialog">
  <div class="modal-dialog modal-dialog-scrollable modal-dialog-centered" role="document">
    <div class="modal-content">
      <!-- content -->
    </div>
  </div>
</div>

Using Bootstrap's scrollable and centered classes keeps modal content contained and stable, reducing layout shifts and improving user experience.

📈 Performance GainSingle reflow on modal open, minimal CLS, smooth scrolling inside modal.

Creating a modal that is scrollable and vertically centered on the screen

Bootsrap

<div class="modal" tabindex="-1" role="dialog">
  <div class="modal-dialog" role="document">
    <div class="modal-content">
      <!-- content -->
    </div>
  </div>
</div>

This modal is not scrollable and not vertically centered, causing content overflow and layout shifts when content is large.

📉 Performance CostTriggers multiple reflows on content overflow and causes CLS due to layout shifts.

Performance Comparison

Pattern	DOM Operations	Reflows	Paint Cost	Verdict
Basic modal without scrollable or centered classes	Low (few nodes)	Multiple reflows on large content overflow	High due to layout shifts	[X] Bad
Modal with modal-dialog-scrollable and modal-dialog-centered classes	Low (few nodes)	Single reflow on open	Low paint cost with stable layout	[OK] Good

Rendering Pipeline

When a scrollable and centered modal opens, the browser calculates styles, layouts the modal dialog centered vertically, and enables internal scrolling without affecting the page layout.

→Style Calculation

→Layout

→Paint

→Composite

⚠️ BottleneckLayout stage due to vertical centering and scroll container sizing

Core Web Vital Affected

CLS

This affects page interaction speed and visual stability when modals appear and scroll within the viewport.

Optimization Tips

1Use modal-dialog-scrollable to keep modal content scrollable inside the viewport.

2Use modal-dialog-centered to vertically center modals and avoid layout shifts.

3Avoid forcing page scroll when modal content overflows to reduce reflows and CLS.

Performance Quiz - 3 Questions

What is the main performance benefit of using Bootstrap's modal-dialog-scrollable class?

A It enables scrolling inside the modal without causing page layout shifts.

B It reduces the modal's file size.

C It disables scrolling on the entire page.

D It automatically lazy loads modal content.

DevTools: Performance

How to check: Open DevTools, go to Performance tab, record while opening the modal with large content, then analyze Layout and Paint events.

What to look for: Look for minimal layout thrashing and stable paint times; check for CLS score in Lighthouse for visual stability.

Practice

1. What does the Bootstrap class modal-dialog-scrollable do in a modal?

easy

A. Disables modal animations

B. Makes the modal content scrollable inside the popup

C. Changes the modal background color

D. Centers the modal vertically on the screen

Solution

1. Step 1: Understand the class purpose

   The class modal-dialog-scrollable is designed to allow the modal content area to scroll if the content is too long.

2. Step 2: Compare with other classes

   Unlike modal-dialog-centered which centers the modal, this class specifically controls scrolling inside the modal.

3. Final Answer:

   Makes the modal content scrollable inside the popup -> Option B

4. Quick Check:

   modal-dialog-scrollable = scrollable content [OK]

Hint: Scrollable modals use modal-dialog-scrollable class [OK]

Common Mistakes:

• Confusing scrollable with centered behavior
• Thinking it changes modal size
• Assuming it affects modal background

2. Which is the correct way to add both scrollable and vertically centered behavior to a Bootstrap modal dialog?

easy

A. <div class='modal-dialog modal-dialog-scrollable modal-dialog-centered'>

B. <div class='modal-dialog scrollable-centered'>

C. <div class='modal-dialog scrollable centered'>

D. <div class='modal-dialog scrollable centered-modal'>

Solution

1. Step 1: Identify correct Bootstrap classes

   Bootstrap uses modal-dialog-scrollable and modal-dialog-centered as separate classes to add scroll and vertical centering.

2. Step 2: Check syntax for multiple classes

   Multiple classes are added separated by spaces inside the class attribute, so both classes must be included exactly as documented.

3. Final Answer:

   <div class='modal-dialog modal-dialog-scrollable modal-dialog-centered'> -> Option A

4. Quick Check:

   Use both classes separated by space [OK]

Hint: Use both classes separated by spaces inside class attribute [OK]

Common Mistakes:

• Using incorrect class names like 'scrollable' or 'centered'
• Combining classes into one word
• Missing one of the required classes

3. Given this modal HTML snippet, what will be the visual effect?

<div class='modal-dialog modal-dialog-scrollable modal-dialog-centered'>
  <div class='modal-content'>
    <div class='modal-body'>
      <p>Very long content here...</p>
    </div>
  </div>
</div>

medium

A. Modal is centered but content does not scroll

B. Modal content scrolls but modal is aligned to top

C. Modal content scrolls inside and modal is vertically centered on screen

D. Modal neither scrolls nor centers vertically

Solution

1. Step 1: Analyze classes used

   The modal uses both modal-dialog-scrollable and modal-dialog-centered classes.

2. Step 2: Understand combined effect

   These classes together make the modal vertically centered and allow the modal body content to scroll if it is long.

3. Final Answer:

   Modal content scrolls inside and modal is vertically centered on screen -> Option C

4. Quick Check:

   Both scrollable and centered classes active = scroll + center [OK]

Hint: Both classes together scroll content and center modal [OK]

Common Mistakes:

• Assuming scroll disables centering
• Thinking content scrolls outside modal
• Confusing modal-body scrolling with page scrolling

4. You added modal-dialog-scrollable but the modal content is not scrolling. What is the likely mistake?

medium

A. You forgot to add modal-dialog-centered

B. You must add scrollable class instead

C. You need to add overflow-auto to modal-content

D. You placed modal-dialog-scrollable on the wrong element

Solution

1. Step 1: Check where the class is applied

   The modal-dialog-scrollable class must be on the modal-dialog element, not on modal-content or others.

2. Step 2: Understand Bootstrap structure

   If placed incorrectly, the scroll behavior won't activate because Bootstrap styles target the dialog container.

3. Final Answer:

   You placed modal-dialog-scrollable on the wrong element -> Option D

4. Quick Check:

   modal-dialog-scrollable must be on modal-dialog [OK]

Hint: Apply scrollable class only on modal-dialog div [OK]

Common Mistakes:

• Adding scrollable to modal-content instead
• Confusing scrollable with centered class
• Forgetting to add modal-dialog class

5. You want a modal that is vertically centered, scrollable, and also has a fixed header and footer that stay visible while the body scrolls. Which Bootstrap structure and classes should you use?

hard

A. Use modal-dialog modal-dialog-scrollable modal-dialog-centered with modal-header, modal-body, and modal-footer inside modal-content

B. Use modal-dialog modal-dialog-centered only and add custom CSS for scrolling

C. Use modal-dialog-scrollable on modal-content and add fixed position to header/footer

D. Use modal-dialog modal-dialog-scrollable but omit modal-header and modal-footer

Solution

1. Step 1: Identify required classes for scroll and center

   Use both modal-dialog-scrollable and modal-dialog-centered on the modal-dialog element.

2. Step 2: Structure modal content properly

   Inside modal-content, include modal-header and modal-footer which remain fixed, while modal-body scrolls.

3. Step 3: Confirm Bootstrap default behavior

   Bootstrap automatically keeps header and footer visible when using scrollable modals with this structure.

4. Final Answer:

   Use modal-dialog modal-dialog-scrollable modal-dialog-centered with modal-header, modal-body, and modal-footer inside modal-content -> Option A

5. Quick Check:

   Combine scrollable + centered + header/footer structure [OK]

Hint: Use both classes on modal-dialog and include header/footer inside modal-content [OK]

Common Mistakes:

• Adding scrollable to modal-content instead of modal-dialog
• Omitting modal-header or modal-footer
• Trying to fix header/footer with custom CSS unnecessarily