Max width ignored for CSS flex.

CSS flex elements seem to handle CSS max-width differently than normal.
For example, the max width might be completely ignored.
I asked Gemini AI about this and got a partially useful answer, shown below.
Further investigation provided more useful information, which I have added afterwards.

I am using CSS flex with max width, but the max width is being exceeded.

※ I am using gap too.

Understanding the Issue: Max Width Exceeded in Flex Container with Gap

Problem: You’re using CSS Flexbox with max-width and gap, but the flex items are exceeding the specified max-width.

Possible Causes:

1. Incorrect flex Property:

   • Ensure that flex-grow is set to 0 or a low value to prevent items from expanding beyond their content size.
   • Use flex-basis to define the initial size of flex items.
   • Example: flex: 0 0 auto;

2. gap Property Impact:

   • The gap property adds space between flex items, potentially increasing the overall width.
   • Consider adjusting the max-width to accommodate the added space.
   • Example: If you have a gap of 20px, you might need to reduce the max-width by the total gap between items.

3. Missing or Incorrect width:

   • If you haven’t explicitly set a width for flex items, they might expand to fill the available space.
   • Set a specific width or use flex-basis to control the initial size.

4. Parent Container Constraints:

   • The parent container might not have a defined width or max-width, allowing flex items to overflow.
   • Ensure the parent container has appropriate size constraints.

5. Other CSS Properties:

   • Other CSS properties like margin, padding, or box-sizing can affect the final size of elements.
   • Check for conflicting styles.

Solution Steps:

1. Provide Code Snippet: Share your HTML and CSS code for a more accurate diagnosis.
2. Check Flex Properties: Verify that flex-grow is set correctly and flex-basis is defined appropriately.
3. Account for gap: Adjust the max-width to compensate for the added space between items.
4. Set Explicit Width: If necessary, provide a specific width for flex items.
5. Inspect Parent Container: Ensure the parent container has proper dimensions.
6. Consider Other CSS: Check for conflicting styles that might affect element size.

Example:

.container {
  display: flex;
  max-width: 800px;
  gap: 20px;
}

.item {
  flex: 0 0 auto; /* Prevent growth and set initial size to content */
  max-width: 200px; /* Adjust max-width considering gap */
}

Additional Tips:

• Use browser developer tools to inspect element dimensions and identify the issue visually.
• Consider using flex-wrap if you want items to wrap to a new line when they exceed the container’s width.

It seems that:

flex items no longer exist in a block formatting context, where block level elements automatically take width: 100%. In a flex formatting context, elements have different defaults.

Therefore, to get max-width to work on a flex item, also give the item width: 100%.

https://stackoverflow.com/a/43705979/1764521

Also, there is an issue with min-width:

Flexbox introduces auto as the initial value of min-width. When overflow is visible, that auto forces the flex item to grow so that its content doesn’t overflow.

Therefore, you can use min-width: 0.

https://stackoverflow.com/a/31972181/1764521