ui: make viewer tab title configurable via templates

We had account and composer tab title configuration fields already, but
not for viewer. Add tab-title-viewer configuration, which defaults to
Subject if it is not empty and to "(no subject)" when it is empty.

Changelog-added: New `[ui].tab-title-viewer` setting to configure the
 message viewer tab title.
Changelog-changed: Message viewer tab titles will now show `(no subject)`
 if there is no subject in the viewed email.
Signed-off-by: Bence Ferdinandy <bence@ferdinandy.com>
Acked-by: Robin Jarry <robin@jarry.cc>

commands/account/view.go

@@ -1,10 +1,14 @@
 package account
 
 import (
+	"bytes"
 	"errors"
 
 	"git.sr.ht/~rjarry/aerc/app"
 	"git.sr.ht/~rjarry/aerc/lib"
+	"git.sr.ht/~rjarry/aerc/lib/state"
+	"git.sr.ht/~rjarry/aerc/lib/templates"
+	"git.sr.ht/~rjarry/aerc/models"
 )
 
 type ViewMessage struct {
@@ -48,7 +52,18 @@ func (v ViewMessage) Execute(args []string) error {
 				return
 			}
 			viewer := app.NewMessageViewer(acct, view)
-			app.NewTab(viewer, msg.Envelope.Subject)
+			data := state.NewDataSetter()
+			data.SetAccount(acct.AccountConfig())
+			data.SetFolder(acct.Directories().SelectedDirectory())
+			data.SetHeaders(msg.RFC822Headers, &models.OriginalMail{})
+			var buf bytes.Buffer
+			err = templates.Render(acct.UiConfig().TabTitleViewer, &buf,
+				data.Data())
+			if err != nil {
+				acct.PushError(err)
+				return
+			}
+			app.NewTab(viewer, buf.String())
 		})
 	return nil
 }

config/ui.go

@@ -83,6 +83,7 @@ type UIConfig struct {
 	// Tab Templates
 	TabTitleAccount  *template.Template `ini:"tab-title-account" default:"{{.Account}}"`
 	TabTitleComposer *template.Template `ini:"tab-title-composer" default:"{{.Subject}}"`
+	TabTitleViewer   *template.Template `ini:"tab-title-viewer" default:"{{if .Subject}}{{.Subject}}{{else}}(no subject){{end}}"`
 
 	// private
 	contextualUis    []*UiConfigContext

doc/aerc-config.5.scd

@@ -195,6 +195,12 @@ These options are configured in the *[ui]* section of _aerc.conf_.
 
 	Default: _{{.Subject}}_
 
+*tab-title-viewer* = _<go_template>_
+	The template to use for viewer tab titles. See *aerc-templates*(7) for
+	available field names.
+
+	Default: _{{if .Subject}}{{.Subject}}{{else}}(no subject){{end}}_
+
 *pinned-tab-marker* = _"<string>"_
 	Marker to show before a pinned tab's name.