Selector: Leverage the :scope pseudo-class where possible

The `:scope` pseudo-class[1] has surprisingly good browser support: Chrome,
Firefox & Safari have supported if for a long time; only IE & Edge lack support.
This commit leverages this pseudo-class to get rid of the ID hack in most cases.
Adding a temporary ID may cause layout thrashing which was reported a few times
in [the past.

We can't completely eliminate the ID hack in modern browses as sibling selectors
require us to change context to the parent and then `:scope` stops applying to
what we'd like. But it'd still improve performance in the vast majority of
cases.

[1] https://developer.mozilla.org/en-US/docs/Web/CSS/:scope

Fixes gh-4453
Closes gh-4454
Ref gh-4332
Ref jquery/sizzle#405

1 files changed, 17 insertions, 0 deletions

diff --git a/src/selector/support.js b/src/selector/support.js
new file mode 100644
index 000000000..86cd2d9ae
--- /dev/null
+++ b/src/selector/support.js
@@ -0,0 +1,17 @@
+define( [
+	"../var/document",
+	"../var/support"
+], function( document, support ) {
+
+"use strict";
+
+// Support: IE 9 - 11+, Edge 12 - 18+
+// IE/Edge don't support the :scope pseudo-class.
+try {
+	document.querySelectorAll( ":scope" );
+	support.scope = true;
+} catch ( e ) {}
+
+return support;
+
+} );