Restore preventDefault, but use tabindex to focus the parent element.

[mbostock]

Work-in-progress. Supersedes #1322. Should fix #1321 #1288 #1099.

Tasks:

• Applying the drag behavior, zoom behavior or brush should set the tabindex attribute and outline style on the nearest HTML ancestor element (the ownerSVGElement when used within SVG, and the current element when used within HTML).
• If the tabindex attribute is already set, it should be preserved? What about the outline style? It might be safe to ignore this possibility.
• The mousedown event should be cancelled (using preventDefault and stopPropagation).
• The touchstart event should not be cancelled for the drag behavior, as this disables draggable links. (Canceling mousedown does not prevent click, but canceling touchstart effectively does.)

[jasondavies]

We should also only bind mouse events for brush on mousedown (not on touchstart), like in #1322. I checked and the same problem happens with this branch (with preventDefault turned back on). I can put it in a separate request if you prefer!

[mbostock]

Feel free to push commits to this branch. But I’m still ruminating over @jfirebaugh’s comments and haven’t decided yet whether I agree. However I am weighing his arguments against the precedent set by other libraries, which seem to favor preventDefault. (Not that we have to agree with every other library, but it’s empirical evidence to consider at least.)

[mbostock]

Closing in favor of preventDefault approach.