Open a thread on its latest message

docs/tui.md

@@ -62,6 +62,8 @@ uppercase belongs to Labels:
 | Ctrl+A | switch linked account |
 | Ctrl+V | choose an Imbox cover |
 
+A thread opens on its latest message; `k` steps back through the ones before it and `j` forward again.
+
 While reading a thread, the From header shows the actual send-as address when HEY records one separately from the account user.
 
 While reading a thread, links can be selected without a mouse. Tab selects the next link in document order and Shift+Tab selects the previous one; both wrap at the ends. A fixed row above the shortcut bar shows the complete selected destination without moving the thread. Press Enter to open it; opening stays unavailable until the terminal is wide enough to show the complete destination. Press Escape once to clear the selection, and Escape again to leave the thread. A thread with no selectable links keeps the normal global Tab focus behavior. Existing OSC 8 mouse links remain available.

internal/tui/mail.go

@@ -591,7 +591,7 @@ func (v *mailView) Update(msg tea.Msg) (tea.Cmd, bool) {
 		}
 		v.imageContent = imageContent.String()
 		v.rebuildTopicContent()
-		v.topicViewport.GotoTop()
+		v.showLatestEntry()
 		// A thread read only in part is not marked seen by being opened: the reader has
 		// not had all of it, and seen would slide it under the Imbox's cover. The seen
 		// key is there for a thread they are done with anyway.
@@ -2202,6 +2202,18 @@ func (v *mailView) currentAttachmentAction(topicID int64, attachmentID string) b
 	return false
 }
 
+// showLatestEntry opens a thread on its newest message, which is where whatever
+// brought the reader here arrived; k walks back through what came before it. The
+// viewport stops at the end of the content, so a short last message shows the tail
+// of the one above it too.
+func (v *mailView) showLatestEntry() {
+	if len(v.entryOffsets) == 0 {
+		v.topicViewport.GotoTop()
+		return
+	}
+	v.topicViewport.SetYOffset(v.entryOffsets[len(v.entryOffsets)-1])
+}
+
 // jumpEntry scrolls the thread to the next or previous message header.
 func (v *mailView) jumpEntry(delta int) {
 	if len(v.entryOffsets) == 0 {

internal/tui/mail_test.go

@@ -294,6 +294,40 @@ func TestMailViewHandlesTopicLoaded(t *testing.T) {
 	}
 }
 
+// A thread opens on its newest message, where whatever brought the reader to it arrived,
+// rather than on the first message of a conversation they have mostly read already.
+func TestOpeningAThreadShowsItsLatestMessage(t *testing.T) {
+	v := mailWithPostings()
+	v.vc.width = 80
+	v.vc.height = 12
+	v.topicViewport.SetWidth(80)
+	v.Update(topicLoadedMsg{
+		boxID:   1,
+		topicID: 100,
+		title:   "Quarterly planning",
+		entries: []mail.Entry{
+			{ID: 1, Creator: mail.Contact{Name: "Maria Gonzalez"}, Body: htmlutil.ToMarkdown(strings.Repeat("<p>Here are the numbers for the quarter.</p>", 8))},
+			{ID: 2, Creator: mail.Contact{Name: "Sam Rivera"}, Body: htmlutil.ToMarkdown(strings.Repeat("<p>Thanks, a few questions on the forecast.</p>", 8))},
+			{ID: 3, Creator: mail.Contact{Name: "Ana Ortiz"}, Body: htmlutil.ToMarkdown(strings.Repeat("<p>Thursday at ten works for me.</p>", 8))},
+		},
+	})
+
+	if len(v.entryOffsets) != 3 {
+		t.Fatalf("entry offsets = %v", v.entryOffsets)
+	}
+	if got, want := v.topicViewport.YOffset(), v.entryOffsets[2]; got != want {
+		t.Errorf("thread opened at line %d, want the latest message at %d", got, want)
+	}
+	if !strings.Contains(ansi.Strip(v.topicViewport.View()), "Ana Ortiz") {
+		t.Errorf("the latest message is not on screen: %q", ansi.Strip(v.topicViewport.View()))
+	}
+
+	v.HandleContentKey(keyPress("k"))
+	if got, want := v.topicViewport.YOffset(), v.entryOffsets[1]; got != want {
+		t.Errorf("k should walk back to the message before it: offset %d, want %d", got, want)
+	}
+}
+
 // A thread heads its messages with the centered subject, and a blank line
 // separates each message header from the content under it.
 func TestThreadViewShowsSubjectAndSpacesTheHeader(t *testing.T) {