rustdoc-search: let people easily make text selections

This is a GUI affordance that I basically copied from DuckDuckGo.
You can make text selections starting in the description by dragging
the mouse, even though clicking on the description opens the page.

src/librustdoc/html/static/js/search.js

@@ -3194,6 +3194,15 @@ ${item.displayPath}<span class="${type}">${name}</span>\
                 li.appendChild(description);
                 li.tabIndex = -1;
                 li.onclick = () => {
+                    // allow user to select the description text without navigating
+                    // also, they can select the path itself by starting the selection here
+                    // (a UI feature I got used to from DuckDuckGo)
+                    if (window.getSelection) {
+                        const selection = window.getSelection();
+                        if (selection && !selection.isCollapsed) {
+                            return;
+                        }
+                    }
                     // allow clicking anywhere on the list item to go to the page
                     // even though the link itself is only the name
                     resultName.click();

tests/rustdoc-gui/search-result-description-selection.goml

@@ -0,0 +1,12 @@
+// This test is to ensure that the codeblocks are correctly rendered in the search results.
+go-to: "file://" + |DOC_PATH| + "/test_docs/index.html?search=some_more_function"
+show-text: true // can't select text without showing it
+// Waiting for the search results to appear...
+wait-for: "#search-tabs"
+assert: ".search-results .desc"
+store-position: (".search-results .desc", {"x": desc_x, "y": desc_y})
+// select text within the description
+// making sure nothing happens after doing so (other than the text select itself)
+drag-and-drop: ((|desc_x| + 8, |desc_y| + 8), (|desc_x| + 80, |desc_y| + 8))
+wait-for: 125
+assert: ".search-results .desc"