[UI] Convert milestone to HTMX

- Currently if you want to update the milestone of an issue or pull
request, your whole page will be reloaded to reflect the newly set
milestone. This is quite unecessary, as only the milestone text is
updated and a new timeline event is added.
- This patch converts the milestone section in the issue/pull request
sidebar to use HTMX, so it becomes a progressive element and avoids
reloading the whole page to update the milestone.
- The update of the milestone section itself is quite straightforward
and nothing special is happening. To support adding new timeline events,
a new element `#insert-timeline` is conviently placed after the last
timeline event, which can be used with
[`hx-swap-oob`](https://htmx.org/attributes/hx-swap-oob/) to position
new timeline events before that element.
- Adds E2E test.

release-notes/4547.md

···
+The milestone section in the sidebar on the issue and pull request page now uses HTMX. If you update the milestone of a issue or pull request it will no longer reload the whole page and instead update the current page with the new information about the milestone update. This should provide a smoother user experience.

routers/web/repo/issue.go

···
 	}
 }
 
+func prepareHiddenCommentType(ctx *context.Context) {
+	var hiddenCommentTypes *big.Int
+	if ctx.IsSigned {
+		val, err := user_model.GetUserSetting(ctx, ctx.Doer.ID, user_model.SettingsKeyHiddenCommentTypes)
+		if err != nil {
+			ctx.ServerError("GetUserSetting", err)
+			return
+		}
+		hiddenCommentTypes, _ = new(big.Int).SetString(val, 10) // we can safely ignore the failed conversion here
+	}
+
+	ctx.Data["ShouldShowCommentType"] = func(commentType issues_model.CommentType) bool {
+		return hiddenCommentTypes == nil || hiddenCommentTypes.Bit(int(commentType)) == 0
+	}
+}
+
 // ViewIssue render issue view page
 func ViewIssue(ctx *context.Context) {
 	if ctx.Params(":type") == "issues" {
···
 	ctx.Data["NewPinAllowed"] = pinAllowed
 	ctx.Data["PinEnabled"] = setting.Repository.Issue.MaxPinned != 0
 
-	var hiddenCommentTypes *big.Int
-	if ctx.IsSigned {
-		val, err := user_model.GetUserSetting(ctx, ctx.Doer.ID, user_model.SettingsKeyHiddenCommentTypes)
-		if err != nil {
-			ctx.ServerError("GetUserSetting", err)
-			return
-		}
-		hiddenCommentTypes, _ = new(big.Int).SetString(val, 10) // we can safely ignore the failed conversion here
-	}
-	ctx.Data["ShouldShowCommentType"] = func(commentType issues_model.CommentType) bool {
-		return hiddenCommentTypes == nil || hiddenCommentTypes.Bit(int(commentType)) == 0
+	prepareHiddenCommentType(ctx)
+	if ctx.Written() {
+		return
 	}
+
 	// For sidebar
 	PrepareBranchList(ctx)
-
 	if ctx.Written() {
 		return
 	}
···
 		}
 	}
 
-	ctx.JSONOK()
+	if ctx.FormBool("htmx") {
+		renderMilestones(ctx)
+		if ctx.Written() {
+			return
+		}
+		prepareHiddenCommentType(ctx)
+		if ctx.Written() {
+			return
+		}
+
+		issue := issues[0]
+		var err error
+		if issue.MilestoneID > 0 {
+			issue.Milestone, err = issues_model.GetMilestoneByRepoID(ctx, ctx.Repo.Repository.ID, issue.MilestoneID)
+			if err != nil {
+				ctx.ServerError("GetMilestoneByRepoID", err)
+				return
+			}
+		} else {
+			issue.Milestone = nil
+		}
+
+		comment := &issues_model.Comment{}
+		has, err := db.GetEngine(ctx).Where("issue_id = ? AND type = ?", issue.ID, issues_model.CommentTypeMilestone).OrderBy("id DESC").Limit(1).Get(comment)
+		if !has || err != nil {
+			ctx.ServerError("GetLatestMilestoneComment", err)
+		}
+		if err := comment.LoadMilestone(ctx); err != nil {
+			ctx.ServerError("LoadMilestone", err)
+			return
+		}
+		if err := comment.LoadPoster(ctx); err != nil {
+			ctx.ServerError("LoadPoster", err)
+			return
+		}
+		issue.Comments = issues_model.CommentList{comment}
+
+		ctx.Data["Issue"] = issue
+		ctx.Data["HasIssuesOrPullsWritePermission"] = ctx.Repo.CanWriteIssuesOrPulls(issue.IsPull)
+		ctx.HTML(http.StatusOK, "htmx/milestone_sidebar")
+	} else {
+		ctx.JSONOK()
+	}
 }
 
 // UpdateIssueAssignee change issue's or pull's assignee

templates/htmx/milestone_sidebar.tmpl

···
+<div id="insert-timeline" hx-swap-oob="beforebegin">
+		 {{template "repo/issue/view_content/comments" .}}
+</div>
+{{template "repo/issue/view_content/sidebar/milestones" .}}

templates/repo/issue/milestone/select_menu.tmpl

···
 	</div>
 	<div class="divider"></div>
 {{end}}
-<div class="no-select item">{{ctx.Locale.Tr "repo.issues.new.clear_milestone"}}</div>
+<div class="no-select item" hx-post="{{$.RepoLink}}/issues/milestone?issue_ids={{$.Issue.ID}}&htmx=true">{{ctx.Locale.Tr "repo.issues.new.clear_milestone"}}</div>
 {{if and (not .OpenMilestones) (not .ClosedMilestones)}}
 	<div class="disabled item">
 		{{ctx.Locale.Tr "repo.issues.new.no_items"}}
···
 			{{ctx.Locale.Tr "repo.issues.new.open_milestone"}}
 		</div>
 		{{range .OpenMilestones}}
-			<a class="item" data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}">
+			<a class="item" hx-post="{{$.RepoLink}}/issues/milestone?id={{.ID}}&issue_ids={{$.Issue.ID}}&htmx=true">
 				{{svg "octicon-milestone" 16 "tw-mr-1"}}
 				{{.Name}}
 			</a>
···
 			{{ctx.Locale.Tr "repo.issues.new.closed_milestone"}}
 		</div>
 		{{range .ClosedMilestones}}
-			<a class="item" data-id="{{.ID}}" data-href="{{$.RepoLink}}/issues?milestone={{.ID}}">
+			<a class="item" hx-post="{{$.RepoLink}}/issues/milestone?id={{.ID}}&issue_ids={{$.Issue.ID}}&htmx=true">
 				{{svg "octicon-milestone" 16 "tw-mr-1"}}
 				{{.Name}}
 			</a>

templates/repo/issue/view_content.tmpl

···
 				</div>
 			</div>
 
-			{{template "repo/issue/view_content/comments" .}}
+		    {{template "repo/issue/view_content/comments" .}}
+			<div id="insert-timeline"></div>
 
 			{{if and .Issue.IsPull (not $.Repository.IsArchived)}}
 				{{template "repo/issue/view_content/pull".}}

templates/repo/issue/view_content/sidebar/milestones.tmpl

···
-<div class="ui {{if or (not .HasIssuesOrPullsWritePermission) .Repository.IsArchived}}disabled{{end}} floating jump select-milestone dropdown">
-	<a class="text muted flex-text-block">
-		<strong>{{ctx.Locale.Tr "repo.issues.new.milestone"}}</strong>
-		{{if and .HasIssuesOrPullsWritePermission (not .Repository.IsArchived)}}
-			{{svg "octicon-gear" 16 "tw-ml-1"}}
-		{{end}}
-	</a>
-	<div class="menu" data-action="update" data-issue-id="{{$.Issue.ID}}" data-update-url="{{$.RepoLink}}/issues/milestone">
-		{{template "repo/issue/milestone/select_menu" .}}
+<div id="milestone-section" hx-swap="morph" hx-target="this" hx-indicator="this">
+	<div class="ui {{if or (not .HasIssuesOrPullsWritePermission) .Repository.IsArchived}}disabled{{end}} floating jump select-milestone dropdown">
+		<a class="text muted flex-text-block">
+			<strong>{{ctx.Locale.Tr "repo.issues.new.milestone"}}</strong>
+			{{if and .HasIssuesOrPullsWritePermission (not .Repository.IsArchived)}}
+				{{svg "octicon-gear" 16 "tw-ml-1"}}
+			{{end}}
+		</a>
+		<div class="menu">
+			{{template "repo/issue/milestone/select_menu" .}}
+		</div>
 	</div>
-</div>
-<div class="ui select-milestone list">
-	<span class="no-select item {{if .Issue.Milestone}}tw-hidden{{end}}">{{ctx.Locale.Tr "repo.issues.new.no_milestone"}}</span>
-	<div class="selected">
-		{{if .Issue.Milestone}}
-			<a class="item muted sidebar-item-link" href="{{.RepoLink}}/milestone/{{.Issue.Milestone.ID}}">
-				{{svg "octicon-milestone" 18 "tw-mr-2"}}
-				{{.Issue.Milestone.Name}}
-			</a>
-		{{end}}
+	<div class="ui select-milestone list">
+		<span class="no-select item {{if .Issue.Milestone}}tw-hidden{{end}}">{{ctx.Locale.Tr "repo.issues.new.no_milestone"}}</span>
+		<div class="selected">
+			{{if .Issue.Milestone}}
+				<a class="item muted sidebar-item-link" href="{{.RepoLink}}/milestone/{{.Issue.Milestone.ID}}">
+					{{svg "octicon-milestone" 18 "tw-mr-2"}}
+					{{.Issue.Milestone.Name}}
+				</a>
+			{{end}}
+		</div>
 	</div>
 </div>

tests/e2e/issue-sidebar.test.e2e.js

···
   await expect(labelList.filter({hasText: 'label2'})).not.toBeVisible();
   await expect(labelList.filter({hasText: 'label1'})).toBeVisible();
 });
+
+test('Issue: Milestone', async ({browser}, workerInfo) => {
+  test.skip(workerInfo.project.name === 'Mobile Safari', 'Unable to get tests working on Safari Mobile, see https://codeberg.org/forgejo/forgejo/pulls/3445#issuecomment-1789636');
+  const page = await login({browser}, workerInfo);
+
+  const response = await page.goto('/user2/repo1/issues/1');
+  await expect(response?.status()).toBe(200);
+
+  const selectedMilestone = page.locator('.issue-content-right .select-milestone.list');
+  const milestoneDropdown = page.locator('.issue-content-right .select-milestone.dropdown');
+  await expect(selectedMilestone).toContainText('No milestone');
+
+  // Add milestone.
+  await milestoneDropdown.click();
+  await page.getByRole('option', {name: 'milestone1'}).click();
+  await expect(selectedMilestone).toContainText('milestone1');
+  await expect(page.locator('.timeline-item.event').last()).toContainText('user2 added this to the milestone1 milestone');
+
+  // Clear milestone.
+  await milestoneDropdown.click();
+  await page.getByText('Clear milestone', {exact: true}).click();
+  await expect(selectedMilestone).toContainText('No milestone');
+  await expect(page.locator('.timeline-item.event').last()).toContainText('user2 removed this from the milestone1 milestone');
+});

web_src/js/features/repo-legacy.js

···
       }
 
       let icon = '';
-      if (input_id === '#milestone_id') {
-        icon = svg('octicon-milestone', 18, 'tw-mr-2');
-      } else if (input_id === '#project_id') {
+      if (input_id === '#project_id') {
         icon = svg('octicon-project', 18, 'tw-mr-2');
       } else if (input_id === '#assignee_id') {
         icon = `<img class="ui avatar image tw-mr-2" alt="avatar" src=${$(this).data('avatar')}>`;
···
 
   // Milestone, Assignee, Project
   selectItem('.select-project', '#project_id');
-  selectItem('.select-milestone', '#milestone_id');
   selectItem('.select-assignee', '#assignee_id');
 }