fix: Fix issue target branch selection for non-collaborators (#36916)

This PR fixes a bug in the UI that prevented non-collaborator users (the
issue poster or creator) from setting the target branch (ref) of an
issue. The backend API already supports this, but the UI was rigidly
disabling the dropdown based only on collaborator status.

Changes:
- Enable the branch selector for the issue poster and during new issue
creation.
- Fix a typo (.IsIssueWriter -> .IsIssuePoster) that was preventing the
reference update URL from being correctly set for posters.

tests/integration/issue_test.go

@@ -680,6 +680,59 @@ func TestUpdateIssueDeadline(t *testing.T) {
 	assert.True(t, issueAfter.DeadlineUnix.IsZero())
 }
 
+func TestUpdateIssueRefByPoster(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	// user4 is a non-admin, non-collaborator on user2/repo1.
+	// They create an issue, making them the poster.
+	posterSession := loginUser(t, "user4")
+	issueURL := testNewIssue(t, posterSession, "user2", "repo1", "Poster ref test", "body")
+	refURL := issueURL + "/ref"
+
+	// The poster (non-collaborator) must be able to update the ref.
+	req := NewRequestWithValues(t, "POST", refURL, map[string]string{"ref": "refs/heads/main"})
+	posterSession.MakeRequest(t, req, http.StatusOK)
+
+	// A different non-collaborator non-poster must be forbidden.
+	otherSession := loginUser(t, "user5")
+	req = NewRequestWithValues(t, "POST", refURL, map[string]string{"ref": "refs/heads/main"})
+	otherSession.MakeRequest(t, req, http.StatusForbidden)
+}
+
+func TestIssueRefSelectorEnabledForPoster(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	// user4 creates an issue in user2/repo1 (user4 has no write permission there).
+	posterSession := loginUser(t, "user4")
+	issueURL := testNewIssue(t, posterSession, "user2", "repo1", "Ref selector test", "body")
+
+	resp := posterSession.MakeRequest(t, NewRequest(t, "GET", issueURL), http.StatusOK)
+	htmlDoc := NewHTMLParser(t, resp.Body)
+
+	// The branch selector must not carry the "disabled" CSS class for the poster.
+	sel := htmlDoc.Find(".branch-selector-dropdown")
+	assert.Equal(t, 1, sel.Length())
+	assert.False(t, sel.HasClass("disabled"), "branch selector should be enabled for the issue poster")
+	// The update-ref URL must be present so JS can send the POST request.
+	_, hasURL := sel.Attr("data-url-update-issueref")
+	assert.True(t, hasURL, "data-url-update-issueref must be set for the issue poster")
+}
+
+func TestIssueRefSelectorEnabledForNewIssue(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	// user4 (non-collaborator on user2/repo1) must see an enabled ref selector
+	// when creating a new issue.
+	session := loginUser(t, "user4")
+	req := NewRequest(t, "GET", "/user2/repo1/issues/new")
+	resp := session.MakeRequest(t, req, http.StatusOK)
+	htmlDoc := NewHTMLParser(t, resp.Body)
+
+	sel := htmlDoc.Find(".branch-selector-dropdown")
+	assert.Equal(t, 1, sel.Length())
+	assert.False(t, sel.HasClass("disabled"), "branch selector should be enabled on the new issue form")
+}
+
 func TestIssueReferenceURL(t *testing.T) {
 	defer tests.PrepareTestEnv(t)()
 	session := loginUser(t, "user2")