Detailed Explanation of CSS position: sticky Property

Description
position: sticky is a hybrid positioning mode where an element behaves as a combination of relative and fixed positioning during scrolling. When the element scrolls to a specific threshold (set via top, right, bottom, or left) within the viewport, it "sticks" to the edge of the viewport from the normal document flow until its parent container leaves the viewport. It is commonly used to achieve sticky header effects for navigation bars or table of contents.

Solution Process

1. Basic Definition and Threshold Setting

   • After setting position: sticky, at least one threshold (e.g., top: 0) must be specified.
   • Example: If an element is set to top: 10px, the sticky effect triggers when the distance between the top of the element and the top of the viewport becomes ≤10px during scrolling.

2. Trigger Conditions for Sticky Behavior

   • Parent Container Limitation: The sticky element remains sticky until the boundary of its parent container touches the opposite edge of the sticky direction (e.g., if top: 0 is set, the sticky effect ends when the parent container's bottom reaches the top of the viewport).
   • Scroll Direction: The sticky effect only works when scrolling in the same direction as the threshold (e.g., top corresponds to vertical downward scrolling).

3. Layer and Stacking Control

   • A sticky element creates a new stacking context, similar to position: relative.
   • If multiple sticky elements overlap, their display priority can be controlled using z-index.

4. Common Issues and Considerations

   • Parent Container Overflow Hidden: If the parent container has overflow: hidden set, the sticky effect will be disabled.
   • Table Element Adaptation: When used on <th> or <tr> elements, position: sticky must be applied to the table cell rather than the row.
   • Browser Compatibility: Check for support in older browsers (e.g., Safari requires the -webkit-sticky prefix).

Example Code

.sticky-header {
  position: sticky;
  top: 0;        /* Threshold for triggering stickiness */
  background: #fff;
  z-index: 100;  /* Ensures the element overlays other content when floating */
}

This code ensures the element remains fixed at the top of the viewport when scrolling downward, making it suitable for sticky header effects in navigation bars.