What you need to know
A large z-index only orders elements inside their stacking context. Properties such as transform, opacity, isolation, and positioned elements can create contexts that limit how descendants stack against other parts of the page.
How to apply it
Inspect the ancestors of the hidden element, find which stacking contexts contain the competing layers, and define a small layer scale for menus, dialogs, and tooltips.
Common mistake to avoid
Do not keep increasing z-index to a huge number. That can hide the real parent context and create fragile ordering across the app.