[css-spatial] Behavior of popups / dropdowns / top-layer elements in spatial contexts #138

Context

The CSS Spatial Layout Module introduces 3D spatial contexts via spatial: page and spatial: portal. Elements within these contexts can be lifted off the page (inset-back/inset-front) and transformed in true 3D space using the user's actual perspective.

However, the spec does not define how popup-style UI behaves when its trigger element (or an ancestor) is spatially positioned and/or transformed. This is a significant gap, as popups, dropdowns, tooltips, dialogs, and native UI are ubiquitous on the web.

Open Questions

1. Author-implemented popups (position: absolute)

If a trigger element is spatially positioned and rotated:

.trigger {
  position: relative;
  inset-back: 2cm;
  transform: rotateX(30deg);
  transform-style: preserve-3d;
}
.menu {
  position: absolute;
  top: 100%;
}

• Does .menu inherit the 3D rendering context and render in the same rotated plane?
• If preserve-3d is not set, is the menu flattened onto the trigger's plane?
• Can the menu use inset-back/inset-front to position itself in z relative to the trigger's plane?

2. position: fixed popups

Spatial containers establish a containing volume. Does this cause position: fixed descendants to be trapped within the spatial context (similar to how transform ancestors break fixed positioning)? Or do fixed elements escape to the page plane?

3. Top-layer elements (<dialog>, popover, :popover)

Top-layer elements currently render above all normal stacking contexts and are not affected by ancestor transforms. In a spatial context:

• Does a <dialog> or popover opened from within a spatial container remain in the top layer (2D, facing the user), or does it enter the 3D spatial context?
• If it enters the spatial context, does it follow the trigger element's rotation, or does it auto-billboard (always face the user)?
• How does anchor positioning (position-area, anchor()) interact with 3D-transformed anchors? The spec introduces position-context: anchor to let elements follow their anchor's transform — is this the intended mechanism for popups?

4. UA-native UI (<select> dropdown, alert(), <input type="date"> picker)

These are rendered by the user agent, not by page CSS:

• Does a <select> inside a rotated spatial container open its dropdown in 2D at the page level, or does it follow the element's spatial transform?
• Does alert()/confirm() remain a 2D browser chrome dialog, or is it spatial-aware?

5. Hit-testing and event coordinates

Section 5 states that precise coordinates must not be transmitted for spatial pointer interactions. How does this affect popups that rely on MouseEvent coordinates (e.g., context menus, drag-to-position tooltips)?

Suggested Next Steps

It would be helpful to have a dedicated section in the spec addressing:

• Which popup mechanisms enter the spatial context vs. remain in the 2D top layer
• Whether an "auto-billboard" behavior is needed for popups to remain readable
• How position-context: anchor should be used for popup positioning in 3D
• Interaction with the existing CSS Top Layer spec and CSS Anchor Positioning spec

Related

• Spec: https://webkit.github.io/explainers/css-spatial/