keyboard accessibility

[dave-doty]

Make all controls keyboard accessible:

https://www.w3.org/TR/WCAG21/#keyboard-accessible

[dave-doty]

One difficulty is that many of the current listeners listen for clicks on SVG elements, but there's no notion of keyboard pressing on an SVG element. So it would need to define some notion of the "currently active" element (distinct from the existing notion of selecting objects, since only one object should be active at a time), and then listening document-wide for a keyboard event and dispatching an Action based on the key code and currently active element.

[dave-doty]

Switching to this SVG pan zoom library would help since it supports keyboard panning: https://github.com/anvaka/panzoom