Clarify interactivity and visibility

[maxkfranz]

Internal APIs are updated to make the distinction of { whether an element takes up space, whether an element is visible, whether an element is interactive } clear, separate, and stable:

• display : Whether to display the element; may be element for displayed or none for not displayed.
  • A display: none element does not take up space.
    • A display: none bundled bezier edge does not take up space in its bundle.
    • A display: none node hides its connected edges.
  • A display: none element is not interactive.
• visibility : Whether the element is visible; may be visible or hidden.
  • A visibility: hidden element does take up space.
    • A visibility: hidden bundled bezier edge does take up space in its bundle.
    • A visibility: hidden node does not hide its connected edges.
  • A visibility: hidden element is not interactive.
• opacity : The opacity of the element, ranging from 0 to 1. Note that the opacity of a compound node parent affects the effective opacity of its children.
  • An opacity: 0 element does take up space.
    • An opacity: 0 bundled bezier edge does take up space in its bundle.
    • An opacity: 0 node does not hide its connected edges.
  • An opacity: 0 element is interactive.

The events: yes | no property does not affect compound children based on how the parent is set. This allows more granular control.

• events : Whether events should occur on an element (e.g. tap, mouseover, etc.); may be yes or no. For no, the element receives no events and events simply pass through to the core/viewport. The events property is per-element, so the value of a compound parent does not affect its children.