Add GUI test for source code viewer scroll handling

src/test/rustdoc-gui/source-anchor-scroll.goml

@@ -0,0 +1,20 @@
+// We check that when the anchor changes and is output of the displayed content,
+// the page is scrolled to it.
+goto: file://|DOC_PATH|/src/link_to_definition/lib.rs.html
+
+// We reduce the window size to make it easier to make an element "out of the page".
+size: (600, 800)
+// We check that the scroll is at the top first.
+assert-property: ("html", {"scrollTop": "0"})
+
+click: '//a[text() = "barbar"]'
+assert-property: ("html", {"scrollTop": "125"})
+click: '//a[text() = "bar"]'
+assert-property: ("html", {"scrollTop": "166"})
+click: '//a[text() = "sub_fn"]'
+assert-property: ("html", {"scrollTop": "53"})
+
+// We now check that clicking on lines doesn't change the scroll
+// Extra information: the "sub_fn" function header is on line 1.
+click: '//*[@id="6"]'
+assert-property: ("html", {"scrollTop": "53"})

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

@@ -1,6 +1,35 @@
+pub fn sub_fn() {
+    barbar();
+}
+fn barbar() {
+    bar(vec![], vec![], vec![], vec![], Bar { a: "a".into(), b: 0 });
+}
+
 pub struct Bar {
     pub a: String,
     pub b: u32,
 }
 
 pub fn foo(_b: &Bar) {}
+
+// The goal now is to add
+// a lot of lines so
+// that the next content
+// will be out of the screen
+// to allow us to test that
+// if the anchor changes to
+// something outside of the
+// current view, it'll
+// scroll to it as expected.
+
+// More filling content.
+
+pub fn bar(
+  _a: Vec<String>,
+  _b: Vec<String>,
+  _c: Vec<String>,
+  _d: Vec<String>,
+  _e: Bar,
+) {
+    sub_fn();
+}