Column stacking on mobile in Bootsrap - Browser Rendering Trace

Render Flow - Column stacking on mobile

Load HTML with Bootstrap classes

↓

Parse Bootstrap CSS

↓

Apply grid classes to columns

↓

Detect viewport width

↓

If viewport < breakpoint: stack columns vertically

↓

If viewport >= breakpoint: arrange columns horizontally

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Paint layout

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Composite final render

The browser loads the HTML and Bootstrap CSS, then applies grid classes to columns. It checks the screen width and stacks columns vertically on small screens (mobile) and arranges them side-by-side on larger screens.

Render Steps - 3 Steps

Code Added:<div class="container"> <div class="row"> <div>Column 1</div> <div>Column 2</div> </div> </div>

Before

→

After

[container]
  [row]
    [Column 1]
    [Column 2]

Basic HTML structure with container, row, and two divs for columns. No Bootstrap grid classes yet, so columns stack by default.

🔧 Browser Action:Creates DOM tree and applies default block layout

Code Sample

Two columns side-by-side on medium and larger screens, stacked vertically on smaller screens like mobile.

Bootsrap

<div class="container">
  <div class="row">
    <div class="col-md-6">Column 1</div>
    <div class="col-md-6">Column 2</div>
  </div>
</div>

Render Quiz - 3 Questions

After applying col-md-6 classes (render_step 2), how do the columns appear on a desktop screen?

A Two columns stacked vertically, each full width

B One column taking full width, the other hidden

C Two columns side-by-side, each taking half the width

D Columns overlap each other

Common Confusions - 2 Topics

Why do my columns stack vertically on mobile even though I used col-md-6?

Because col-md-6 applies the half-width only on medium screens and larger. On smaller screens, columns default to full width and stack vertically. This is shown in render_steps 2 and 3.

💡 Bootstrap grid classes apply from their breakpoint and up; below that, columns stack.

Why doesn't adding col-md-6 make columns side-by-side on mobile?

The 'md' in col-md-6 means the style applies starting at medium screen sizes (≥768px). On mobile (smaller than 768px), the columns stack vertically by default.

💡 Use col- or col-sm- classes to control layout on smaller screens.

Property Reference

Property/Class	Breakpoint	Effect on Columns	Visual Behavior	Common Use
col	None (always)	Columns share equal width	Columns side-by-side always	Simple equal columns on all screens
col-md-6	≥768px (medium)	Columns take 50% width on md and up	Side-by-side on md+, stacked on smaller	Responsive two-column layout
col-sm-12	≥576px (small)	Columns take full width on sm and up	Stacked on small and below	Force stacking on small screens
row	N/A	Creates horizontal flex container	Aligns columns horizontally by default	Wrap columns in grid row

Concept Snapshot

Bootstrap columns stack vertically on small screens by default. Use col-md-6 to make columns side-by-side on medium (≥768px) and larger screens. Below the md breakpoint, columns take full width and stack. The 'row' class creates a flex container for horizontal layout. Resize viewport to see columns stack or align side-by-side responsively.

Practice

1. What does the Bootstrap class col-12 do on mobile screens?

easy

A. Makes the column take full width and stack vertically

B. Makes the column take half width and stay side-by-side

C. Hides the column on mobile screens

D. Centers the column horizontally

Solution

1. Step 1: Understand col-12 meaning

   The class col-12 means the column will take all 12 parts of the grid, which is full width.

2. Step 2: Effect on mobile screens

   On small screens, columns with col-12 stack vertically because each takes full width.

3. Final Answer:

   Makes the column take full width and stack vertically -> Option A

4. Quick Check:

   col-12 = full width stack [OK]

Hint: Full width columns stack vertically on mobile with col-12 [OK]

Common Mistakes:

• Thinking col-12 means half width
• Assuming columns stay side-by-side on mobile
• Confusing col-12 with hiding columns

2. Which Bootstrap class combination makes two columns stack on mobile but sit side-by-side on medium screens?

easy

A. col-6 col-md-12

B. col-md-6 col-lg-12

C. col-6 col-lg-6

D. col-12 col-md-6

Solution

1. Step 1: Analyze mobile stacking

   col-12 makes columns full width on mobile, so they stack vertically.

2. Step 2: Analyze medium screen layout

   col-md-6 makes columns half width on medium screens, so they sit side-by-side.

3. Final Answer:

   col-12 col-md-6 -> Option D

4. Quick Check:

   Mobile stack = col-12, Medium side-by-side = col-md-6 [OK]

Hint: Use col-12 for mobile stack, col-md-6 for medium side-by-side [OK]

Common Mistakes:

• Reversing col-12 and col-md-6 classes
• Using col-6 on mobile which prevents stacking
• Ignoring breakpoint order in class names

3. Given this HTML snippet:

<div class='row'>
  <div class='col-12 col-md-4'>A</div>
  <div class='col-12 col-md-8'>B</div>
</div>

How will columns appear on a small mobile screen?

medium

A. Columns A and B stacked vertically, each full width

B. Columns A and B stacked vertically, widths 4/12 and 8/12

C. Only column A visible, column B hidden

D. Columns A and B side-by-side with widths 4/12 and 8/12

Solution

1. Step 1: Check mobile classes

   Both columns have col-12 which means full width on small screens, so they stack vertically.

2. Step 2: Check medium screen classes

   col-md-4 and col-md-8 apply only on medium and larger screens, so no effect on mobile.

3. Final Answer:

   Columns A and B stacked vertically, each full width -> Option A

4. Quick Check:

   Mobile uses col-12, so stacked full width [OK]

Hint: On mobile, col-12 stacks columns vertically regardless of col-md-* [OK]

Common Mistakes:

• Thinking col-md-* affects mobile layout
• Assuming side-by-side on mobile
• Confusing widths on small screens

4. You want two columns to stack on mobile but sit side-by-side on medium screens. You wrote:

<div class='row'>
  <div class='col-md-6'>Left</div>
  <div class='col-md-6'>Right</div>
</div>

Why do the columns NOT stack on mobile?

medium

A. Because col-md-6 applies to mobile screens too, so on mobile columns default to equal width side-by-side

B. Because you forgot to add col-12 to make columns full width on mobile

C. Because row class disables stacking on mobile

D. Because columns need col-sm-12 to stack on mobile

Solution

1. Step 1: Understand default column behavior

   Without col-* classes for small screens, columns try to share space equally side-by-side.

2. Step 2: Add col-12 for mobile stacking

   Adding col-12 makes columns full width on mobile, forcing vertical stacking.

3. Final Answer:

   You forgot to add col-12 to make columns full width on mobile -> Option B

4. Quick Check:

   Mobile stack needs col-12 class [OK]

Hint: Add col-12 for mobile stacking, col-md-6 for medium side-by-side [OK]

Common Mistakes:

• Assuming col-md-6 controls mobile layout
• Thinking row class prevents stacking
• Using col-sm-12 instead of col-12 for mobile

5. You want a layout with three columns that stack on mobile, show two columns side-by-side on tablets, and three columns side-by-side on desktops. Which class setup achieves this?

hard

A. col-12 col-sm-4 col-lg-4

B. col-6 col-md-4 col-lg-3

C. col-12 col-md-6 col-lg-4

D. col-12 col-md-4 col-lg-3

Solution

1. Step 1: Mobile stacking with full width

   col-12 makes columns full width on mobile, so they stack vertically.

2. Step 2: Two columns side-by-side on tablets

   col-md-6 means on medium screens (tablets), columns take half width, so two fit side-by-side.

3. Step 3: Three columns side-by-side on desktops

   col-lg-4 means on large screens (desktops), columns take 4/12 width, so three fit side-by-side.

4. Final Answer:

   col-12 col-md-6 col-lg-4 -> Option C

5. Quick Check:

   Mobile stack = col-12, Tablet 2 cols = col-md-6, Desktop 3 cols = col-lg-4 [OK]

Hint: Use col-12 mobile, col-md-6 tablet, col-lg-4 desktop for stacking [OK]

Common Mistakes:

• Using col-sm instead of col-md for tablets
• Choosing wrong column widths that don't add up
• Forgetting col-12 for mobile stacking