Tests: Backport the `hidden="until-found"` attr tests from 3.x-stableHEADmain

The `hidden` attribute used to be a boolean one but it gained a new
`until-found` eventually. This led us to change the way we handle boolean
attributes in jQuery 4.0 in gh-5452 to avoid these issues in the future.

We haven't added an explicit test for the `"until-found"` value of the
`hidden` attribute which triggered this decision so far, though.
Backport the test from gh-5607 which landed on `3.x-stable` so that we
do test it.

Closes gh-5619
Ref gh-5452
Ref gh-5607

(cherry picked from commit 85290c59724db69a3967885892d4305d76de058f)

1 files changed, 18 insertions, 0 deletions

diff --git a/test/unit/attributes.js b/test/unit/attributes.js
index a54f2d65d..5ace087b9 100644
--- a/test/unit/attributes.js
+++ b/test/unit/attributes.js
@@ -479,6 +479,24 @@ QUnit.test( "attr(String, Object)", function( assert ) {
 	assert.equal( jQuery( "#name" ).attr( "nonexisting", undefined ).attr( "nonexisting" ), undefined, ".attr('attribute', undefined) does not create attribute (trac-5571)" );
 } );
 
+QUnit.test( "attr( previously-boolean-attr, non-boolean-value)", function( assert ) {
+	assert.expect( 3 );
+
+	var div = jQuery( "<div></div>" ).appendTo( "#qunit-fixture" );
+
+	div.attr( "hidden", "foo" );
+	assert.strictEqual( div.attr( "hidden" ), "foo",
+		"Values not normalized for previously-boolean hidden attribute" );
+
+	div.attr( "hidden", "until-found" );
+	assert.strictEqual( div.attr( "hidden" ), "until-found",
+		"`until-found` value preserved for hidden attribute" );
+
+	div.attr( "hiDdeN", "uNtil-fOund" );
+	assert.strictEqual( div.attr( "hidden" ), "uNtil-fOund",
+		"`uNtil-fOund` different casing preserved" );
+} );
+
 QUnit.test( "attr(non-ASCII)", function( assert ) {
 	assert.expect( 2 );