Why Doesn't My Sticky Element with `bottom: 0` Appear to Stick?

Understanding CSS Sticky Positioning and Its Behavior with top and bottom

CSS's sticky positioning allows an element to remain fixed within its containing block until a specific threshold is crossed, where it then becomes relatively positioned. However, there appears to be a discrepancy when using 'bottom: 0' with sticky elements.

Problem Explanation:

Consider the following code:

<div class="block">
  <div class="move">
  </div>
</div>

.block {
  background: pink;
  width: 50%;
  height: 200px;
}

.move {
  position: sticky;
  bottom: 0;
}

When 'move' is set to 'top:0', it sticks to the top of the pink block, behaving as expected. However, when 'move' is set to 'bottom:0', it appears to become detached and no longer remains fixed.

Understanding the behavior of 'bottom: 0' with sticky elements:

To understand this behavior, it's crucial to grasp the definition of sticky positioning:

A stickily positioned element remains relatively positioned until its containing block crosses a specified threshold (such as setting 'top' to a value other than 'auto'), after which it becomes 'stuck' until meeting the opposite edge of its containing block.

In the case of 'bottom: 0', the sticky element is not visible because it is stuck to the bottom edge of its containing block. As the scroll position passes the threshold, the containing block moves up, but the sticky element remains fixed to the bottom edge. Since the sticky element is already at the bottom, it cannot move any further upwards.

To observe the sticky behavior, ensure the containing block has sufficient height to accommodate the sticky element. By providing a significant top margin to the block element, you can scroll through the page and see the sticky element adhere to the bottom

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