Use first commit title for multi-commit PRs and fix auto-focus title field (#36606)

Fixes: https://github.com/go-gitea/gitea/issues/34865

1. When opening a PR from a branch with multiple commits, use the first
(oldest) commit's title as the default title instead of the branch name
2. Fix autofocus on PR title input field

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

routers/web/repo/compare_test.go

@@ -4,9 +4,16 @@
 package repo
 
 import (
+	"strings"
 	"testing"
+	"unicode/utf8"
 
+	asymkey_model "code.gitea.io/gitea/models/asymkey"
+	git_model "code.gitea.io/gitea/models/git"
 	issues_model "code.gitea.io/gitea/models/issues"
+	user_model "code.gitea.io/gitea/models/user"
+	"code.gitea.io/gitea/modules/git"
+	git_service "code.gitea.io/gitea/services/git"
 	"code.gitea.io/gitea/services/gitdiff"
 
 	"github.com/stretchr/testify/assert"
@@ -38,3 +45,47 @@ func TestAttachCommentsToLines(t *testing.T) {
 	assert.Equal(t, int64(300), section.Lines[1].Comments[0].ID)
 	assert.Equal(t, int64(301), section.Lines[1].Comments[1].ID)
 }
+
+func TestNewPullRequestTitleContent(t *testing.T) {
+	ci := &git_service.CompareInfo{HeadRef: "refs/heads/head-branch"}
+
+	mockCommit := func(msg string) *git_model.SignCommitWithStatuses {
+		return &git_model.SignCommitWithStatuses{
+			SignCommit: &asymkey_model.SignCommit{
+				UserCommit: &user_model.UserCommit{
+					Commit: &git.Commit{
+						CommitMessage: msg,
+					},
+				},
+			},
+		}
+	}
+
+	title, content := prepareNewPullRequestTitleContent(ci, nil)
+	assert.Equal(t, "head-branch", title)
+	assert.Empty(t, content)
+
+	title, content = prepareNewPullRequestTitleContent(ci, []*git_model.SignCommitWithStatuses{mockCommit("title-only")})
+	assert.Equal(t, "title-only", title)
+	assert.Empty(t, content)
+
+	title, content = prepareNewPullRequestTitleContent(ci, []*git_model.SignCommitWithStatuses{mockCommit("title-" + strings.Repeat("a", 255))})
+	assert.Equal(t, "title-aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa…", title)
+	assert.Equal(t, "…aaaaaaaaa\n", content)
+
+	title, content = prepareNewPullRequestTitleContent(ci, []*git_model.SignCommitWithStatuses{mockCommit("title\nbody")})
+	assert.Equal(t, "title", title)
+	assert.Equal(t, "body", content)
+
+	title, content = prepareNewPullRequestTitleContent(ci, []*git_model.SignCommitWithStatuses{mockCommit("a\xf0\xf0\xf0\nb\xf0\xf0\xf0")})
+	assert.Equal(t, "a?", title) // FIXME: GIT-COMMIT-MESSAGE-ENCODING: "title" doesn't use the same charset converting logic as "content"
+	assert.Equal(t, "b"+string(utf8.RuneError)+string(utf8.RuneError), content)
+
+	title, content = prepareNewPullRequestTitleContent(ci, []*git_model.SignCommitWithStatuses{
+		// ordered from newest to oldest
+		mockCommit("title2\nbody2"),
+		mockCommit("title1\nbody1"),
+	})
+	assert.Equal(t, "title1", title)
+	assert.Empty(t, content)
+}