What is z-index in CSS?
Z-index is a CSS property that controls the stacking order of positioned elements on a page. It determines which element appears on top when multiple elements visually overlap.
Why does z-index matter in UI development?
Z-index is critical because incorrect values can cause overlays, menus, or modals to appear behind other elements. This can make key interactions impossible to use and create confusing visual glitches for users.
Which elements commonly use z-index?
Headers, dropdown menus, modals, tooltips, sticky navigation bars, and notification banners often rely on carefully set z-index values to ensure they appear above the main content when needed.
How is a z-index value interpreted?
Within the same stacking context, elements with higher z-index values stack above those with lower values. If two elements overlap and share the same context, the one with the higher z-index will be visible on top.
What is a stacking context?
A stacking context is a grouping of elements whose z-index ordering is calculated together and separately from other parts of the page. Each stacking context maintains its own internal “stacking world,” independent of elements outside it.
What causes unexpected z-index issues?
Unexpected issues often occur when new stacking contexts are created unintentionally—for example, through CSS properties like position with certain values, transform, opacity, or filter. These can isolate elements and make z-index changes seem ineffective.
How can z-index problems be debugged?
Browser developer tools help inspect elements, see their stacking context, and review applied z-index values. By examining these layers and temporarily adjusting styles, developers can identify where conflicts arise and how to resolve them.
Who works with z-index most often?
Front-end developers and UI engineers work with z-index frequently as part of layout and interaction implementation. They ensure that interactive components layer correctly across different devices and viewports.
How can design systems handle z-index?
Design systems often define a standardised z-index scale—for example, base content, dropdowns, overlays, and modals—each with predefined values. This prevents ad hoc numbers and keeps stacking behaviour consistent across the entire interface.
Why does z-index indirectly affect UX and conversion?
When z-index is handled well, critical components like menus, forms, and modals are always visible and usable. This reduces friction, prevents user frustration, and protects key actions—such as sign-ups, checkouts, or demo requests—from being blocked by layout issues.
