[css-ui] Consider adding a new property for disabling context menu on long press on mobile

[upsuper]

On mobile browsers, long pressing certain kinds of elements would trigger a context menu to show up, this is sometimes undesired.

One case of that is for dragging target, that users may not have decided where to drag to before they start pressing, and the popup context menu may break such work flow.

iOS has -webkit-touch-callout property specifically for handling this, but it's not available elsewhere.

There are several suggested workarounds for this on Android, like invoking preventDefault on contextmenu event, or maybe even touchstart event, or using pointer-events: none for the specific elements. But they all involve some trade-offs so may not fit in all the cases.

Given these, I think it's probably good to have a standard property for controlling this, and I guess CSS UI spec is where such property should be in.

[fantasai]

Is this stylistic or semantic? :) Like, does it belong in CSS or HTML seems like a legit question to ask here...

[upsuper]

That's a good question. For whether it's stylistic or semantic, I don't have a strong feeling either way. I would say it's probably in the same category as user-select.

From the use case side, I think developers would generally want to apply this to a certain type of elements based on their tag and/or position in the tree (e.g. <img> and <a> are elements which usually have the behavior, and drag target may be wrapped in certain kind of elements or themselves have certain classes applied), so it might be more convenient to use in CSS than in HTML.

[fantasai]

There are a lot of situations in which semantic attributes would be "more convenient to use" with CSS syntax rather than HTML syntax. But that seems like more of an argument for HTML adopting some kind of CSS syntax than making HTML attributes be CSS properties.

[upsuper]

So how to know whether this is stylistic or semantic? Like, is there a guideline for judging whether a functionality should belong to CSS or HTML?

[frivoal]

It's not a silver bullet, but I'd start by asking whether it is more tightly coupled with stuff that's in css, or with stuff that's in HTML: is it something you're likely to want to change if you're restyling the whole page or not? Should it work in reader mode?

[upsuper]

In that sense, I guess it is probably more stylistic.

For example, you may only want to disable such context menu when it can be interacted with, e.g. in editing mode, and when you are presenting the content without editing ability, you may want it to behave as normal element. In this specific example, whether being in editing mode would be semantic (e.g. controlled by HTML attribute contenteditable), and the behavior of how specific element respond to the user action can be considered stylistic.

Does this make sense?

When it comes to interactive behavior, it feels that the boundary between stylistic and semantic isn't that clear...