<details>/<summary> UI fixes

src/librustdoc/html/static/css/rustdoc.css

@@ -158,8 +158,8 @@ h1.fqn {
 	Underlines elsewhere in the documentation break up visual flow and tend to invert
 	section hierarchies. */
 h2,
-.top-doc h3,
-.top-doc h4 {
+.top-doc .docblock > h3,
+.top-doc .docblock > h4 {
 	border-bottom: 1px solid;
 }
 h3.code-header {
@@ -1669,6 +1669,11 @@ details.rustdoc-toggle[open] > summary.hideme::after {
 	content: "Collapse";
 }
 
+/* This is needed in docblocks to have the "▶" element to be on the same line. */
+.docblock summary > * {
+	display: inline-block;
+}
+
 /* Media Queries */
 
 @media (min-width: 701px) {

src/test/rustdoc-gui/docblock-details.goml

@@ -0,0 +1,23 @@
+// This ensures that the `<details>`/`<summary>` elements are displayed as expected.
+goto: file://|DOC_PATH|/test_docs/details/struct.Details.html
+show-text: true
+local-storage: {"rustdoc-theme": "dark", "rustdoc-use-system-theme": "false"}
+reload:
+
+// We first check that the headers in the `.top-doc` doc block still have their
+// bottom border.
+assert-text: (".top-doc .docblock > h3", "Hello")
+assert-css: (
+    ".top-doc .docblock > h3",
+    {"border-bottom": "1px solid rgb(221, 221, 221)"},
+)
+// We now check that the `<summary>` doesn't have a bottom border and has the correct display.
+assert-css: (
+    ".top-doc .docblock summary h4",
+    {"border-bottom": "0px none rgb(221, 221, 221)"},
+)
+// This allows to ensure that summary is on one line only!
+assert-property: (".top-doc .docblock summary h4", {"offsetHeight": "33"})
+assert-css: (".top-doc .docblock summary h4", {"margin-top": "15px", "margin-bottom": "5px"})
+// So `33 + 15 + 5` == `53`
+assert-property: (".top-doc .docblock summary", {"offsetHeight": "53"})

src/test/rustdoc-gui/src/test_docs/lib.rs

@@ -277,3 +277,15 @@ pub use macros::*;
 
 #[doc(alias = "AliasForTheStdReexport")]
 pub use ::std as TheStdReexport;
+
+pub mod details {
+    /// We check the appearance of the `<details>`/`<summary>` in here.
+    ///
+    /// ## Hello
+    ///
+    /// <details>
+    /// <summary><h4>I'm a summary</h4></summary>
+    /// <div>I'm the content of the details!</div>
+    /// </details>
+    pub struct Details;
+}