msgviewer: Add Labels as a virtual header

In order to accommodate for that, the headerlayout needed to be rewritten
to pass the filter criteria back to the msgviewer, instead of just using
the normal headers.

widgets/headerlayout.go

@@ -7,15 +7,19 @@ import (
 
 type HeaderLayout [][]string
 
+type HeaderLayoutFilter struct {
+	layout HeaderLayout
+	keep   func(msg *models.MessageInfo, header string) bool // filter criteria
+}
+
 // forMessage returns a filtered header layout, removing rows whose headers
 // do not appear in the provided message.
-func (layout HeaderLayout) forMessage(msg *models.MessageInfo) HeaderLayout {
-	headers := msg.RFC822Headers
-	result := make(HeaderLayout, 0, len(layout))
-	for _, row := range layout {
+func (filter HeaderLayoutFilter) forMessage(msg *models.MessageInfo) HeaderLayout {
+	result := make(HeaderLayout, 0, len(filter.layout))
+	for _, row := range filter.layout {
 		// To preserve layout alignment, only hide rows if all columns are empty
 		for _, col := range row {
-			if headers.Get(col) != "" {
+			if filter.keep(msg, col) {
 				result = append(result, row)
 				break
 			}

widgets/msgviewer.go

@@ -52,7 +52,16 @@ type PartSwitcher struct {
 func NewMessageViewer(acct *AccountView, conf *config.AercConfig,
 	store *lib.MessageStore, msg *models.MessageInfo) *MessageViewer {
 
-	layout := HeaderLayout(conf.Viewer.HeaderLayout).forMessage(msg)
+	hf := HeaderLayoutFilter{
+		layout: HeaderLayout(conf.Viewer.HeaderLayout),
+		keep: func(msg *models.MessageInfo, header string) bool {
+			if fmtHeader(msg, header, "2") != "" {
+				return true
+			}
+			return false
+		},
+	}
+	layout := hf.forMessage(msg)
 	header, headerHeight := layout.grid(
 		func(header string) ui.Drawable {
 			return &HeaderView{
@@ -109,6 +118,8 @@ func fmtHeader(msg *models.MessageInfo, header string, timefmt string) string {
 		return msg.Envelope.Date.Local().Format(timefmt)
 	case "Subject":
 		return msg.Envelope.Subject
+	case "Labels":
+		return strings.Join(msg.Labels, ", ")
 	default:
 		return msg.RFC822Headers.Get(header)
 	}
@@ -580,6 +591,11 @@ func (pv *PartViewer) attemptCopy() {
 						"%s: %s\n", fields.Key(), fields.Value())
 					pv.sink.Write([]byte(field))
 				}
+				// virtual header
+				if len(pv.msg.Labels) != 0 {
+					labels := fmtHeader(pv.msg, "Labels", "")
+					pv.sink.Write([]byte(fmt.Sprintf("Labels: %s\n", labels)))
+				}
 				pv.sink.Write([]byte{'\n'})
 			}