Fix incorrect URL for "Reference in New Issue" (go-gitea#28716)

Gitea prefers to use relative URLs in code (to make multiple domain work
for some users)

So it needs to use `toAbsoluteUrl` to generate a full URL when click
"Reference in New Issues"

And add some comments in the test code

tests/integration/issue_test.go

@@ -607,3 +607,22 @@ func TestUpdateIssueDeadline(t *testing.T) {
 
 	assert.EqualValues(t, "2022-04-06", apiIssue.Deadline.Format("2006-01-02"))
 }
+
+func TestIssueReferenceURL(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+	session := loginUser(t, "user2")
+
+	issue := unittest.AssertExistsAndLoadBean(t, &issues_model.Issue{ID: 1})
+	repo := unittest.AssertExistsAndLoadBean(t, &repo_model.Repository{ID: issue.RepoID})
+
+	req := NewRequest(t, "GET", fmt.Sprintf("%s/issues/%d", repo.FullName(), issue.Index))
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	htmlDoc := NewHTMLParser(t, resp.Body)
+
+	// the "reference" uses relative URLs, then JS code will convert them to absolute URLs for current origin, in case users are using multiple domains
+	ref, _ := htmlDoc.Find(`.timeline-item.comment.first .reference-issue`).Attr("data-reference")
+	assert.EqualValues(t, "/user2/repo1/issues/1#issue-1", ref)
+
+	ref, _ = htmlDoc.Find(`.timeline-item.comment:not(.first) .reference-issue`).Attr("data-reference")
+	assert.EqualValues(t, "/user2/repo1/issues/1#issuecomment-2", ref)
+}

web_src/js/features/repo-issue.js

@@ -4,6 +4,7 @@ import {showTemporaryTooltip, createTippy} from '../modules/tippy.js';
 import {hideElem, showElem, toggleElem} from '../utils/dom.js';
 import {setFileFolding} from './file-fold.js';
 import {getComboMarkdownEditor, initComboMarkdownEditor} from './comp/ComboMarkdownEditor.js';
+import {toAbsoluteUrl} from '../utils.js';
 
 const {appSubUrl, csrfToken} = window.config;
 
@@ -527,7 +528,7 @@ export function initRepoIssueReferenceIssue() {
     const $this = $(this);
     const content = $(`#${$this.data('target')}`).text();
     const poster = $this.data('poster-username');
-    const reference = $this.data('reference');
+    const reference = toAbsoluteUrl($this.data('reference'));
     const $modal = $($this.data('modal'));
     $modal.find('textarea[name="content"]').val(`${content}\n\n_Originally posted by @${poster} in ${reference}_`);
     $modal.modal('show');