There is a complex UI: panels, maps, pop-up windows. Sometimes a dropdown list falls under another element, even though it has the maximum z-index. What could be the fundamental reason for this behavior in CSS and how to fix it in Vue?

The fundamental reason why a dropdown with the highest z-index might appear under another element is the stacking context in CSS. Each stacking context is a separate 'layer' within which its own z-index operates. If the parent of the dropdown is in one stacking context, and another element is in a different, higher one, then even a large z-index within its own context won't help it appear above elements in another context.

Reasons for creating a stacking context:

• An element with positioning (relative, absolute, fixed) and a z-index other than auto
• An element with opacity less than 1
• An element with transform, filter, perspective, etc.
• An element with CSS property isolation: isolate

How to fix in Vue:

1. Find which element creates a higher stacking context and obstructs display.
2. Move the dropdown in the DOM so that it is outside the problematic context, for example, using a portal (vue-portal or Teleport in Vue 3).
3. Ensure that the dropdown and its parents do not have properties that create unexpected stacking contexts.

Example of using Teleport in Vue 3 to render the dropdown at the end of the body:

<template>
  <teleport to="body">
    <div class="dropdown">
      <!-- dropdown content -->
    </div>
  </teleport>
</template>

This guarantees that the dropdown will be in the topmost stacking context and won't be hidden under other elements.