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[csswg-drafts] [css-ui] The computed value rules of user-select is problematic. (#3344)

From: Emilio Cobos Álvarez via GitHub <sysbot+gh@w3.org>
Date: Sun, 25 Nov 2018 23:29:17 +0000
To: public-css-archive@w3.org
Message-ID: <issues.opened-384131178-1543188556-sysbot+gh@w3.org>
emilio has just created a new issue for https://github.com/w3c/csswg-drafts:

== [css-ui] The computed value rules of user-select is problematic. ==
(Note: this is somewhat similar to https://github.com/w3c/csswg-drafts/issues/336)

I've been looking at user-select issues in https://bugzilla.mozilla.org/show_bug.cgi?id=1506547.

The rules for computing the value of user-select as specified in https://drafts.csswg.org/css-ui-4/#propdef-user-select have pretty problematic implications (which is why at least Gecko / Blink / WebKit don't implement it as such):

 * The initial computed value of the property depends on the element. If the element is editable the computed value should be `contain`, but otherwise it should be `text`. That is problematic since there's no canonical "initial computed style" anymore.

 * The computed value of a reset property depends on the parent style: This is problematic performance-wise. This is not unsolvable (`justify-items` has the same issue), but adds a lot of complexity and memory usage. This is worse for `user-select` since the common behavior is to "inherit" unlike `justify-items`. But basically this means that two elements that match the same rules no longer have the same computed style for reset properties, which is the invariant that powers some of Gecko's style engine optimizations.

Gecko / Blink / WebKit do all these checks at used-value time, but the initial value is `auto`. In Gecko the property is not inherited, but in Blink / WebKit it is inherited:


Can we make these checks at used-value time instead, keeping `auto` as a computed value? You may need to walk the tree up anyway because of `user-select: all`, so this seems better.

We could implement this efficiently to some extent in Gecko by making it inherited and adding some hacks to set the default value when the parent style is `contain` and it's not overriden, or something of that sort, but I honestly don't think the complexity is worth it.

So concrete proposal would be:

 * Make `user-select` a non-inherited property (as it is today), with initial **computed and specified** value of `auto`.
 * `auto` means the following at used-value time:
    * If the element is an editable root, then the value is `contain`.
    * If there's no ancestor with a non-`auto` used value, or that `auto` used value is `contain`, then `text`.

Behavior-wise this is identical to the current spec, except `auto` is a valid computed value, and maybe related to `display: contents` depending if we choose to use ancestor box or ancestor element in the flat tree.

cc @frivoal @kojiishi @FremyCompany @rniwa

Please view or discuss this issue at https://github.com/w3c/csswg-drafts/issues/3344 using your GitHub account
Received on Sunday, 25 November 2018 23:29:19 UTC

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