diff --git a/testing/web-platform/tests/html/semantics/popovers/popover-attribute-basic.html b/testing/web-platform/tests/html/semantics/popovers/popover-attribute-basic.html
index 13108949cb3c5..2af3bbc137f1a 100644
--- a/testing/web-platform/tests/html/semantics/popovers/popover-attribute-basic.html
+++ b/testing/web-platform/tests/html/semantics/popovers/popover-attribute-basic.html
@@ -1,6 +1,7 @@
+
diff --git a/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting-anchor.tentative.html b/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting-anchor.tentative.html
new file mode 100644
index 0000000000000..4520ab0577040
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting-anchor.tentative.html
@@ -0,0 +1,45 @@
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Single popover=auto ancestor
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Single popover=manual ancestor
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Nested popover=auto ancestors
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Nested popover=auto ancestors, target is outer
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Top layer inside of nested element
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diff --git a/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting-hints.tentative.html b/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting-hints.tentative.html
new file mode 100644
index 0000000000000..4ec1f49bda9fb
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting-hints.tentative.html
@@ -0,0 +1,46 @@
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Single popover=hint ancestor
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Nested auto/hint ancestors
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Nested auto/hint ancestors, target is auto
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Unrelated hint, target=hint
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Unrelated hint, target=auto
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diff --git a/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting.tentative.html b/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting.tentative.html
new file mode 100644
index 0000000000000..a0b3b60b72b55
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/popovers/popover-top-layer-nesting.tentative.html
@@ -0,0 +1,45 @@
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Single popover=auto ancestor
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Single popover=manual ancestor
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Nested popover=auto ancestors
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Nested popover=auto ancestors, target is outer
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Top layer inside of nested element
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diff --git a/testing/web-platform/tests/html/semantics/popovers/resources/popover-top-layer-nesting.js b/testing/web-platform/tests/html/semantics/popovers/resources/popover-top-layer-nesting.js
new file mode 100644
index 0000000000000..ace10b3f7bc47
--- /dev/null
+++ b/testing/web-platform/tests/html/semantics/popovers/resources/popover-top-layer-nesting.js
@@ -0,0 +1,108 @@
+function createTopLayerElement(t,topLayerType) {
+ let element, show, showing;
+ switch (topLayerType) {
+ case 'dialog':
+ element = document.createElement('dialog');
+ show = () => element.showModal();
+ showing = () => element.matches(':modal');
+ break;
+ case 'fullscreen':
+ element = document.createElement('div');
+ show = async (topmostElement) => {
+ // Be sure to add user activation to the topmost visible target:
+ await blessTopLayer(topmostElement);
+ await element.requestFullscreen();
+ };
+ showing = () => document.fullscreenElement === element;
+ break;
+ default:
+ assert_unreached('Invalid top layer type');
+ }
+ t.add_cleanup(() => element.remove());
+ return {element,show,showing};
+}
+function runTopLayerTests(testCases, testAnchorAttribute) {
+ testAnchorAttribute = testAnchorAttribute || false;
+ testCases.forEach(test => {
+ const description = test.firstChild.data.trim();
+ assert_equals(test.querySelectorAll('.target').length,1,'There should be exactly one target');
+ const target = test.querySelector('.target');
+ assert_true(!!target,'Invalid test case');
+ const popovers = Array.from(test.querySelectorAll('[popover]'));
+ assert_true(popovers.length > 0,'No popovers found');
+ ['dialog','fullscreen'].forEach(topLayerType => {
+ promise_test(async t => {
+ const {element,show,showing} = createTopLayerElement(t,topLayerType);
+ target.appendChild(element);
+
+ // Show the popovers.
+ t.add_cleanup(() => popovers.forEach(popover => popover.hidePopover()));
+ popovers.forEach(popover => popover.showPopover());
+ popovers.forEach(popover => assert_true(popover.matches(':popover-open'),'All popovers should be open'));
+
+ // Activate the top layer element.
+ await show(popovers[popovers.length-1]);
+ assert_true(showing());
+ popovers.forEach(popover => assert_equals(popover.matches(':popover-open'),popover.dataset.stayOpen==='true','Incorrect behavior'));
+
+ // Add another popover within the top layer element and make sure entire stack stays open.
+ const newPopover = document.createElement('div');
+ t.add_cleanup(() => newPopover.remove());
+ newPopover.popover = popoverHintSupported() ? 'hint' : 'auto';
+ element.appendChild(newPopover);
+ popovers.forEach(popover => assert_equals(popover.matches(':popover-open'),popover.dataset.stayOpen==='true','Adding another popover shouldn\'t change anything'));
+ assert_true(showing(),'top layer element should still be top layer');
+ newPopover.showPopover();
+ assert_true(newPopover.matches(':popover-open'));
+ popovers.forEach(popover => assert_equals(popover.matches(':popover-open'),popover.dataset.stayOpen==='true','Showing the popover shouldn\'t change anything'));
+ assert_true(showing(),'top layer element should still be top layer');
+ },`${description} with ${topLayerType}`);
+
+ promise_test(async t => {
+ const {element,show,showing} = createTopLayerElement(t,topLayerType);
+ element.popover = popoverHintSupported() ? 'hint' : 'auto';
+ target.appendChild(element);
+
+ // Show the popovers.
+ t.add_cleanup(() => popovers.forEach(popover => popover.hidePopover()));
+ popovers.forEach(popover => popover.showPopover());
+ popovers.forEach(popover => assert_true(popover.matches(':popover-open'),'All popovers should be open'));
+ const targetWasOpenPopover = target.matches(':popover-open');
+
+ // Show the top layer element as a popover first.
+ element.showPopover();
+ assert_true(element.matches(':popover-open'),'element should be open as a popover');
+ assert_equals(target.matches(':popover-open'),targetWasOpenPopover,'target shouldn\'t change popover state');
+
+ try {
+ await show(element);
+ assert_unreached('It is an error to activate a top layer element that is already a showing popover');
+ } catch (e) {
+ // We expect an InvalidStateError for dialogs, and a TypeError for fullscreens.
+ // Anything else should fall through to the test harness.
+ if (e.name !== 'InvalidStateError' && e.name !== 'TypeError') {
+ throw e;
+ }
+ }
+ },`${description} with ${topLayerType}, top layer element *is* a popover`);
+
+ if (testAnchorAttribute) {
+ promise_test(async t => {
+ const {element,show,showing} = createTopLayerElement(t,topLayerType);
+ element.anchorElement = target;
+ document.body.appendChild(element);
+
+ // Show the popovers.
+ t.add_cleanup(() => popovers.forEach(popover => popover.hidePopover()));
+ popovers.forEach(popover => popover.showPopover());
+ popovers.forEach(popover => assert_true(popover.matches(':popover-open'),'All popovers should be open'));
+
+ // Activate the top layer element.
+ await show(popovers[popovers.length-1]);
+ assert_true(showing());
+ popovers.forEach(popover => assert_equals(popover.matches(':popover-open'),popover.dataset.stayOpen==='true','Incorrect behavior'));
+ },`${description} with ${topLayerType}, anchor attribute`);
+ }
+ });
+ });
+}