preventDefault on mousedown and touchstart by default? about d3-drag

[mbostock]

There’s a partial workaround for the inability to capture events with mousedown.preventDefault: listen to top rather than window. But it only works if the top frame has the same domain as the frame, which makes the workaround much less useful.

[mbostock]

If we mousedown.preventDefault:

• We may want to call element.focus on an ancestor. However, that appears to only work on HTML elements with a tabIndex attribute or SVG a elements. Alternatively we could call element.blur on the document.activeElement.
• We may want to listen to top rather than window. However, this would require catching the security error and a fallback to window, so dragging will still break if the mouse leaves the iframe and the top window is not on the same domain.
• We may or may not want to suppress all clicks, or just clicks on moved drags.

If we don’t mousedown.preventDefault:

• We need to preventDefault on dragstart and selectstart and set-moz-user-select: none.
• We may or may not want to suppress all clicks, or just clicks on moved drags.

If we touchstart.preventDefault:

• We may want to generate a synthetic click, or just on a moved drag. This is a bit tricky. We can’t use element.click because it’s unsupported on SVG elements, and we probably want the click event to have the correct coordinates.

If we don’t touchstart.preventDefault:

• Some drag operations may trigger a pinch-zoom. It’s unclear when or why, as in most cases touchmove.preventDefault seems to be sufficient to prevent it.

[jfirebaugh]

👍

In contrast to D3 3.x, it is not necessary to fork the implementation.

Given this, I agree that it's best to have the most standards compliant and forward looking behavior be the default for D3 4.0.

[mbostock]

Thanks for replying. I think I’m leaning towards keeping things mostly the same with D3 3.x (although D3 4.0 will be more easily customizable, should you want different behavior) for now.

When the Chrome bug is fixed, I might switch to mousedown.preventDefault, but then I’d need to figure out how to focus.

When Pointer Events are supported, we could use those instead of touch events, and then we should be able to pointerdown.preventDefault to prevent pinch-and-zoom without suppressing click.

But it’s all a big if, since the nature of the preventDefault API is that it doesn’t let you explicitly specify what you intend to prevent.

[Herst]

Pointer events have landed in Chrome in the meantime: https://caniuse.com/#feat=pointer

[Herst]

The selectstart event landed in Firefox: https://caniuse.com/#feat=selection-api