Draw diverged commits on the spine as a fork

A tip, checkout or committed pin that has diverged from the pin gets its
own lane, which joins the main lane at a "base" node: the merge base of
the two, with its SHA and subject. Only one diverged commit is drawn as a
fork; with two, or without a merge base, the nodes stay in a line with
the "diverged" gap.

Add Status.Bases, filled by the git provider with `merge-base` for each
diverged commit.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/tui/detail/detail.go

@@ -208,6 +208,20 @@ func (m Model) spineLines(nodes []submodule.SpineNode, offPin bool) []string {
 	pinAt := slices.IndexFunc(nodes, func(n submodule.SpineNode) bool { return hasRole(n, submodule.RolePin) })
 	stagedAt := slices.IndexFunc(nodes, func(n submodule.SpineNode) bool { return hasRole(n, submodule.RoleStaged) })
 
+	// a fork has its own lane to the right of the main one; it starts at the
+	// first node and joins the main lane at the base
+	fork, baseAt := len(nodes) > 0 && nodes[0].Fork, -1
+	if fork {
+		baseAt = slices.IndexFunc(nodes, func(n submodule.SpineNode) bool { return hasRole(n, submodule.RoleBase) })
+	}
+	line := muted.Render(t.Glyphs.Gap)
+	lanes := func(main, side string) string {
+		if !fork {
+			return main
+		}
+		return main + " " + side
+	}
+
 	var out []string
 	for i, n := range nodes {
 		label, sha := muted, theme.Fg(t.Primary)
@@ -217,10 +231,30 @@ func (m Model) spineLines(nodes []submodule.SpineNode, offPin bool) []string {
 		if offPin && len(n.Roles) == 1 && n.Roles[0] == submodule.RoleYou {
 			label, sha = theme.Fg(t.Warn), theme.Fg(t.Warn)
 		}
-		out = append(out, label.Width(width).Render(labels[i])+sha.Render(t.Glyphs.Node+" "+n.Commit.SHA)+" "+text.Render(n.Commit.Subject))
+		node := sha.Render(t.Glyphs.Node)
+		var cells string
+		switch {
+		case n.Fork:
+			cells = lanes(" ", node)
+		case i == baseAt:
+			cells = node + muted.Render(t.Glyphs.Rule+t.Glyphs.Join)
+		case fork && i < baseAt:
+			cells = lanes(node, line)
+		default:
+			cells = lanes(node, " ")
+		}
+		out = append(out, label.Width(width).Render(labels[i])+cells+" "+sha.Render(n.Commit.SHA)+" "+text.Render(n.Commit.Subject))
 		if i < len(nodes)-1 {
-			gap := muted.Render(t.Glyphs.Gap + m.linkText(n.Below))
-			if i+1 == stagedAt && pinAt >= 0 && pinAt < stagedAt && n.Below.Linear() {
+			// the main lane starts below the fork's head, the fork's lane ends at the base
+			main, side := " ", " "
+			if !fork || i > 0 {
+				main = line
+			}
+			if fork && i < baseAt {
+				side = line
+			}
+			gap := lanes(main, side) + muted.Render(m.linkText(n.Below))
+			if !fork && i+1 == stagedAt && pinAt >= 0 && pinAt < stagedAt && n.Below.Linear() {
 				gap += theme.Fg(t.Info).Render(" (staged pin moves back)")
 			}
 			out = append(out, strings.Repeat(" ", width)+gap)

internal/tui/detail/spine_test.go

@@ -73,3 +73,28 @@ func TestSpineLinesInSync(t *testing.T) {
 		t.Errorf("got %q, want %q", got, want)
 	}
 }
+
+func TestSpineLinesFork(t *testing.T) {
+	m := New(theme.Default(), nil)
+	c := func(sha, subject string) *submodule.Commit { return &submodule.Commit{SHA: sha, Subject: subject} }
+	st := submodule.Status{
+		Pinned: c("c4d5e6f", "Experimental"), CheckedOut: c("c4d5e6f", "Experimental"), Tip: c("e1f2a3b", "Drop Go 1.21"),
+		Pin:   submodule.PinInfo{Ahead: 1, Behind: 2},
+		Bases: submodule.Bases{Tip: c("a1b2c3d", "Release 1.4")},
+	}
+	var got []string
+	for _, l := range m.spineLines(st.Spine(), false) {
+		got = append(got, ansi.Strip(l))
+	}
+	g := theme.Default().Glyphs
+	want := []string{
+		"tip       " + "  " + g.Node + " e1f2a3b Drop Go 1.21",
+		"          " + "  " + g.Gap + " 2 commits",
+		"pin · you " + g.Node + " " + g.Gap + " c4d5e6f Experimental",
+		"          " + g.Gap + " " + g.Gap + " 1 commit",
+		"base      " + g.Node + g.Rule + g.Join + " a1b2c3d Release 1.4",
+	}
+	if !slices.Equal(got, want) {
+		t.Errorf("got\n%q\nwant\n%q", got, want)
+	}
+}

internal/tui/theme/issues.go

@@ -41,6 +41,8 @@ func RoleWord(r submodule.Role) string {
 		return "HEAD"
 	case submodule.RoleStaged:
 		return Staged
+	case submodule.RoleBase:
+		return "base"
 	case submodule.RoleTip:
 		return "tip"
 	default:

internal/tui/theme/theme.go

@@ -54,17 +54,17 @@ func Fg(c lipgloss.TerminalColor) lipgloss.Style {
 
 // Glyphs are the few symbols used next to the state words.
 type Glyphs struct {
-	Staged, Ahead, Behind, Collapsed, Expanded, Problems, Minus, From, Rule, Node, Gap string
+	Staged, Ahead, Behind, Collapsed, Expanded, Problems, Minus, From, Rule, Node, Gap, Join string
 }
 
 func Unicode() Glyphs {
 	return Glyphs{
-		Staged: "●", Ahead: "↑", Behind: "↓", Collapsed: "▸", Expanded: "▾", Problems: "·", Minus: "−", From: "←", Rule: "─", Node: "●", Gap: "┊",
+		Staged: "●", Ahead: "↑", Behind: "↓", Collapsed: "▸", Expanded: "▾", Problems: "·", Minus: "−", From: "←", Rule: "─", Node: "●", Gap: "┊", Join: "╯",
 	}
 }
 
 func ASCII() Glyphs {
 	return Glyphs{
-		Staged: "*", Ahead: "^", Behind: "v", Collapsed: ">", Expanded: "v", Problems: ".", Minus: "-", From: "<-", Rule: "-", Node: "o", Gap: ":",
+		Staged: "*", Ahead: "^", Behind: "v", Collapsed: ">", Expanded: "v", Problems: ".", Minus: "-", From: "<-", Rule: "-", Node: "o", Gap: ":", Join: "/",
 	}
 }