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>

docs/decisions.md

@@ -414,3 +414,13 @@ header keeps the progress and `n failed`. `Repo.FetchedAt` is gone.
 **Amendment (with D25).** A staged pin shows both pins on the spine: `staged`
 (the index) and `pin` (the superproject's HEAD, `Status.Committed`), so a
 bump reads as a distance. Before, only the index pin was shown at all.
+
+**Amendment (with D25): forks.** A diverged tip, checkout or committed pin is
+drawn as a second lane that joins the main one at a `base` node (SHA +
+subject of `git merge-base`), instead of a straight line with
+`diverged ↑n ↓m`. The base was chosen over a bare join mark: where the split
+happened is what decides between a merge and a rebase. Cost: one
+`merge-base` and one `log -1` per diverged commit, only when it is diverged.
+Only one fork is drawn; with two diverged commits (rare) the old line with
+the diverged gap remains, since nodes on three lanes would need the
+merge bases between all of them.

docs/requirements.md

@@ -64,6 +64,8 @@ commit hashes. It doesn't answer the questions users actually have:
   **tip** and the checkout (**you**) as one line of history, newest on top:
   a node per commit (SHA + subject) with its roles, and the number of
   commits between two nodes. Roles on the same commit share a node. A
+  commit that has diverged from the pin is drawn as a fork that joins at
+  the commit where the histories split (`base`). A
   checkout that differs from the pin is highlighted.
 - **F22** *(dropped)* The commits between pin and tip are not listed; only
   their number is shown (F21).

docs/state-model.md

@@ -32,6 +32,7 @@ in several at once.
 | `Pin` | `Ref`, `Source`, `Ahead`, `Behind`, `Pushed` |
 | `Pinned`, `CheckedOut`, `Tip` | Commits (SHA, subject, author, time) |
 | `Drift` | Checkout compared with the pin: `Ahead` = commits only the checkout has, `Behind` = commits only the pin has |
+| `Bases` | `Tip`, `Checkout`, `Committed`: `merge-base <pin> <that commit>` for each commit that is diverged from the pin (ahead and behind); nil otherwise or without a common ancestor |
 | `Committed`, `CommittedDrift` | With a staged pin: the pin in the superproject's HEAD (nil if it is not on disk or the submodule was only added) and how it compares with the staged pin, like `Drift` |
 | `Changes` | Changed files of the worktree (nil if clean), see below |
 | `FetchedAt` | Last fetch of the submodule (`FETCH_HEAD` mtime); zero if never |
@@ -174,6 +175,7 @@ are local; `<pin>` is the index gitlink.
 | `Pin.Ref` | rules above; the remote is `origin`, else the first remote |
 | `Ahead`, `Behind` | `rev-list --left-right --count <pin>...<ref>` |
 | `Drift` | `rev-list --left-right --count <head>...<pin>`, only if the checkout differs from the pin |
+| `Bases` | `merge-base <pin> <ref>` / `<head>` / `<committed>`, only when diverged; then `log -1` of it |
 | `Committed`, `CommittedDrift` | `ls-tree HEAD` gitlink, if it differs from the pin and exists locally; `rev-list --left-right --count <committed>...<pin>` |
 
 Details:
@@ -212,10 +214,20 @@ roles (`pin`, `tip`, `you`; `HEAD` for the superproject). It uses `Pin.Ahead` /
   *behind* below it; the `Link` between two nodes counts the commits
   separating them. The count is the distance, so a tip 4 commits ahead
   of the pin shows `4 commits`, and an adjacent one `1 commit`.
-- A diverged tip is placed above the pin, a diverged checkout below it,
-  with a diverged `Link` (`Above` and `Below` count the commits only the
-  upper or lower node has). The link between a diverged node and a node other
-  than the pin is unknown (zero `Link`).
+- A commit that has **diverged** from the pin (tip, checkout or committed
+  pin) is drawn as a **fork** if it is the only one and its merge base is
+  known (`Status.Bases`): it is the first node (`SpineNode.Fork`), with the
+  number of commits down to the base as its `Below`; the main lane follows
+  (pin and the linear nodes), and a `RoleBase` node where the two histories
+  split. Linear nodes between the pin and the base stay on the main lane
+  above it, older ones continue below it, and one that is the base commit
+  shares its node (`you · base`). The pin-to-base distance is the pin's side
+  of the divergence.
+- Otherwise (two diverged commits, or no merge base) there is no fork: a
+  diverged tip is placed above the pin, a diverged checkout below it, with a
+  diverged `Link` (`Above` and `Below` count the commits only the upper or
+  lower node has). The link between a diverged node and a node other than the
+  pin is unknown (zero `Link`).
 - With a staged pin the node of `Pinned` (the index) is labelled `staged`, and
   the committed pin gets its own `pin` node, placed by `CommittedDrift` like
   the checkout. Both can share nodes with `tip` and `you`.

docs/ux-guide.md

@@ -118,9 +118,23 @@ you       ● 48005ac Merge branch 'feat/ethercat…
   sits below it, and the gap above it says `(staged pin moves back)` (blue).
 - The checkout (`you`) is yellow (label and SHA) when it is on its own node,
   i.e. off pin. Everything else is the default text color, the gap muted.
-- A diverged pair shows `┊ diverged ↑n ↓m` (arrows from the lower node's
-  view, like the tree). A real fork is not drawn yet.
-- The nodes are `●` (`o` with `SUBMARINE_ASCII`), the gap `┊` (`:`).
+- A commit that has **diverged** from the pin gets its own lane to the right
+  of the main one. The two lanes join at a `base` node, the commit where the
+  histories split; the counts say how many commits each side has since:
+
+  ```
+  tip         ● e1f2a3b Drop Go 1.21 support
+              ┊ 2 commits
+  pin · you ● ┊ c4d5e6f Experimental HTTP/3
+            ┊ ┊ 1 commit
+  base      ●─╯ a9b8c7d Release 1.9.0
+  ```
+
+  Only one diverged commit is drawn as a fork, and only if the base is known.
+  Otherwise the nodes stay in a line and the gap says `┊ diverged ↑n ↓m`
+  (arrows from the lower node's view, like the tree).
+- The nodes are `●` (`o` with `SUBMARINE_ASCII`), the gap `┊` (`:`), the
+  join `─╯` (`-/`).
 - A missing submodule has only the pin node (SHA, no subject).
 
 #### Change preview

internal/submodule/git/provider.go

@@ -241,9 +241,24 @@ func committedState(ctx context.Context, in info, st *submodule.Status, committe
 		return err
 	}
 	st.Committed = c
+	if st.CommittedDrift.Ahead > 0 && st.CommittedDrift.Behind > 0 {
+		if st.Bases.Committed, err = mergeBase(ctx, in.dir, committed, pin); err != nil {
+			return err
+		}
+	}
 	return nil
 }
 
+// mergeBase returns the commit where the histories of a and b split; nil if
+// they have none in common.
+func mergeBase(ctx context.Context, dir, a, b string) (*submodule.Commit, error) {
+	base, ok, err := try(ctx, dir, "merge-base", a, b)
+	if err != nil || !ok || base == "" {
+		return nil, err
+	}
+	return commit(ctx, dir, base)
+}
+
 // rootStatus describes the superproject. Its "pin" is HEAD, compared with the
 // upstream of the current branch; submodules are ignored, their rows show
 // them.
@@ -346,6 +361,11 @@ func pinState(ctx context.Context, in info, st *submodule.Status, pin, head, bra
 		if st.Drift.Ahead, st.Drift.Behind, err = parseLeftRight(out); err != nil {
 			return err
 		}
+		if st.Drift.Ahead > 0 && st.Drift.Behind > 0 {
+			if st.Bases.Checkout, err = mergeBase(ctx, in.dir, pin, head); err != nil {
+				return err
+			}
+		}
 	}
 	if source == submodule.RefFromSHA {
 		st.Pin.Ref = st.Pinned.SHA
@@ -361,6 +381,11 @@ func pinState(ctx context.Context, in info, st *submodule.Status, pin, head, bra
 	if st.Pin.Ahead, st.Pin.Behind, err = parseLeftRight(out); err != nil {
 		return err
 	}
+	if st.Pin.Ahead > 0 && st.Pin.Behind > 0 {
+		if st.Bases.Tip, err = mergeBase(ctx, in.dir, pin, ref); err != nil {
+			return err
+		}
+	}
 	return nil
 }
 

internal/submodule/git/provider_test.go

@@ -294,3 +294,50 @@ func TestCommittedPinAheadOfStaged(t *testing.T) {
 		t.Errorf("spine = %+v, want the committed pin directly above the staged one", spine)
 	}
 }
+
+// A pin and its ref that each have a commit the other lacks, and a checkout
+// that did the same to the pin: both get the merge base.
+func TestBasesOfDivergedCommits(t *testing.T) {
+	p, root := ownFixture(t)
+	ctx := context.Background()
+	super := filepath.Join(root, "super")
+
+	// src/net: the pin is unpushed; upstream moved on
+	pushUpstream(t, root, "net")
+	if err := p.Fetch(ctx, "src/net"); err != nil {
+		t.Fatal(err)
+	}
+	st, err := p.Status(ctx, "src/net")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Pin.Ahead != 1 || st.Pin.Behind != 1 {
+		t.Fatalf("src/net: ↑%d↓%d, want ↑1↓1", st.Pin.Ahead, st.Pin.Behind)
+	}
+	if b := st.Bases.Tip; b == nil || b.SHA == st.Pinned.SHA || b.SHA == st.Tip.SHA {
+		t.Errorf("src/net: Bases.Tip = %+v, want a commit before the pin and the tip", b)
+	}
+	if sp := st.Spine(); len(sp) != 3 || !sp[0].Fork || sp[0].Commit.SHA != st.Tip.SHA {
+		t.Errorf("src/net: spine = %+v, want a fork headed by the tip", sp)
+	}
+
+	// src/core: a commit on top of a checkout that is behind the pin
+	git(t, filepath.Join(super, "src", "core"), "commit", "--allow-empty", "-m", "Local")
+	st, err = p.Status(ctx, "src/core")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if st.Drift != (submodule.Distance{Ahead: 1, Behind: 1}) {
+		t.Fatalf("src/core: drift %+v, want 1/1", st.Drift)
+	}
+	if b := st.Bases.Checkout; b == nil || b.SHA == st.Pinned.SHA || b.SHA == st.CheckedOut.SHA {
+		t.Errorf("src/core: Bases.Checkout = %+v", b)
+	}
+
+	// in sync and linear: no bases
+	if st, err = p.Status(ctx, "src/ui"); err != nil {
+		t.Fatal(err)
+	} else if st.Bases != (submodule.Bases{}) {
+		t.Errorf("src/ui: Bases = %+v, want none", st.Bases)
+	}
+}

internal/submodule/mock/mock.go

@@ -278,6 +278,7 @@ func fixture() []node {
 		}},
 		{relPath: "libs/http", status: &submodule.Status{
 			Pin:        submodule.PinInfo{Ref: "origin/develop", Ahead: 1, Behind: 2, Pushed: true, Source: submodule.RefFromContains},
+			Bases:      submodule.Bases{Tip: commit("a9b8c7d", "Release 1.9.0", "frank", 20*24*time.Hour)},
 			Pinned:     httpPin,
 			CheckedOut: httpPin,
 			Tip:        httpTip,

internal/submodule/model.go

@@ -176,6 +176,11 @@ func (c *Changes) Sort() {
 // `git rev-list --left-right --count a...b`.
 type Distance struct{ Ahead, Behind int }
 
+// Bases are the merge bases of the pin with the commits that have diverged
+// from it, where the histories split. A field is nil if that commit has not
+// diverged or the base is unknown.
+type Bases struct{ Tip, Checkout, Committed *Commit }
+
 // Status is the full state of one submodule.
 type Status struct {
 	Root       bool // the superproject: the "pin" is its HEAD
@@ -193,6 +198,7 @@ type Status struct {
 	// only added. CommittedDrift compares it with Pinned like Drift.
 	Committed      *Commit
 	CommittedDrift Distance
+	Bases          Bases
 	Changes        *Changes  // nil if the worktree is clean or the details are unknown
 	FetchedAt      time.Time // last fetch of the submodule; zero if never
 }

internal/submodule/spine.go

@@ -12,12 +12,17 @@ const (
 	RoleStaged             // the pin in the index, if it differs from RolePin
 	RoleTip                // the tip of the ref the pin was resolved to
 	RoleYou                // the commit checked out in the submodule
+	RoleBase               // where a diverged commit's history splits from the pin's
 )
 
 // SpineNode is one commit on the spine and the roles that sit on it.
 type SpineNode struct {
 	Roles  []Role
 	Commit *Commit
+	// Fork marks the head of a diverged branch. It is the first node; its
+	// Below is the number of commits down to the RoleBase node, on its own
+	// lane, and the other nodes follow on the main lane.
+	Fork bool
 	// Below describes the history between this node and the next one down;
 	// it is the zero Link for the last node.
 	Below Link
@@ -42,19 +47,26 @@ func (l Link) Diverged() bool { return l.Above > 0 && l.Below > 0 }
 
 // spineItem is a commit with its roles on its way to becoming a node.
 type spineItem struct {
-	roles  []Role
-	commit *Commit
-	height int  // commits above (+) or below (−) the pin; unused if div
-	div    bool // diverged from the pin
-	own    int  // commits only this item has
-	pin    int  // commits only the pin has
-	isPin  bool
+	roles    []Role
+	commit   *Commit
+	height   int     // commits above (+) or below (−) the pin; unused if div
+	div      bool    // diverged from the pin
+	divAbove bool    // where a diverged item goes if it is not drawn as a fork
+	base     *Commit // merge base with the pin, if div and known
+	own      int     // commits only this item has
+	pin      int     // commits only the pin has
+	isPin    bool
 }
 
-// Spine arranges the pin (and the staged pin, if any), the tip and the checkout as one line of history,
-// newest on top. Roles on the same commit share a node. A tip or checkout
-// that has diverged from the pin is placed outermost, above or below
-// respectively, with a diverged Link. Missing commits are left out.
+// Spine arranges the pin (and the staged pin, if any), the tip and the
+// checkout as one line of history, newest on top. Roles on the same commit
+// share a node.
+//
+// A commit that has diverged from the pin is drawn as a fork if it is the
+// only one and its merge base is known: it is the first node, and the
+// RoleBase node where the histories split follows on the main lane. Otherwise
+// a diverged tip is placed above the pin and a diverged checkout below it,
+// with a diverged Link. Missing commits are left out.
 func (s *Status) Spine() []SpineNode {
 	if s.Pinned == nil {
 		return nil
@@ -68,13 +80,12 @@ func (s *Status) Spine() []SpineNode {
 	}
 	pin := spineItem{roles: []Role{pinRole}, commit: s.Pinned, isPin: true}
 
-	var above, below []spineItem
-	place := func(role Role, c *Commit, own, pinOnly int, divAbove bool) {
+	var above, below, divs []spineItem
+	place := func(role Role, c, base *Commit, own, pinOnly int, divAbove bool) {
 		it := spineItem{roles: []Role{role}, commit: c, own: own, pin: pinOnly}
 		switch {
 		case own == 0 && pinOnly == 0:
 			pin.roles = append(pin.roles, role)
-			return
 		case own > 0 && pinOnly == 0:
 			it.height = own
 			above = append(above, it)
@@ -82,26 +93,37 @@ func (s *Status) Spine() []SpineNode {
 			it.height = -pinOnly
 			below = append(below, it)
 		default:
-			it.div = true
-			if divAbove {
-				above = append(above, it)
-			} else {
-				below = append(below, it)
-			}
+			it.div, it.divAbove, it.base = true, divAbove, base
+			divs = append(divs, it)
 		}
 	}
 	if s.Tip != nil {
-		place(RoleTip, s.Tip, s.Pin.Behind, s.Pin.Ahead, true)
+		place(RoleTip, s.Tip, s.Bases.Tip, s.Pin.Behind, s.Pin.Ahead, true)
 	}
 	if s.PinStaged && s.Committed != nil {
-		place(RolePin, s.Committed, s.CommittedDrift.Ahead, s.CommittedDrift.Behind, false)
+		place(RolePin, s.Committed, s.Bases.Committed, s.CommittedDrift.Ahead, s.CommittedDrift.Behind, false)
 	}
 	switch {
 	case s.CheckedOut == nil, s.Root: // HEAD is the superproject's pin and checkout
 	case s.CheckedOut.SHA == s.Pinned.SHA:
 		pin.roles = append(pin.roles, RoleYou)
 	default:
-		place(RoleYou, s.CheckedOut, s.Drift.Ahead, s.Drift.Behind, false)
+		place(RoleYou, s.CheckedOut, s.Bases.Checkout, s.Drift.Ahead, s.Drift.Behind, false)
+	}
+
+	var fork *spineItem
+	if len(divs) == 1 && divs[0].base != nil {
+		fork = &divs[0]
+		// the main lane continues to the base, where the fork joins it
+		below = append(below, spineItem{roles: []Role{RoleBase}, commit: fork.base, height: -fork.pin})
+	} else {
+		for _, it := range divs {
+			if it.divAbove {
+				above = append(above, it)
+			} else {
+				below = append(below, it)
+			}
+		}
 	}
 
 	// diverged items are outermost, the rest is ordered by height
@@ -143,6 +165,10 @@ func (s *Status) Spine() []SpineNode {
 		nodes = append(nodes, SpineNode{Roles: it.roles, Commit: it.commit})
 		last = it
 	}
+	if fork != nil {
+		head := SpineNode{Roles: fork.roles, Commit: fork.commit, Fork: true, Below: Link{Above: fork.own}}
+		nodes = append([]SpineNode{head}, nodes...)
+	}
 	return nodes
 }
 

internal/submodule/spine_test.go

@@ -80,6 +80,48 @@ func TestSpine(t *testing.T) {
 			nodes: []string{"t:tip", "s:staged", "p:pin,you"},
 			links: []Link{{Above: 1}, {Above: 2}},
 		},
+		{
+			name: "tip diverged from the pin: a fork",
+			st: Status{Pinned: c("p"), CheckedOut: c("p"), Tip: c("t"), Pin: PinInfo{Ahead: 1, Behind: 2},
+				Bases: Bases{Tip: c("b")}},
+			nodes: []string{"t:tip", "p:pin,you", "b:base"},
+			links: []Link{{Above: 2}, {Above: 1}},
+		},
+		{
+			name: "checkout diverged from the pin: a fork",
+			st: Status{Pinned: c("p"), CheckedOut: c("y"), Tip: c("p"), Drift: Distance{Ahead: 3, Behind: 2},
+				Bases: Bases{Checkout: c("b")}},
+			nodes: []string{"y:you", "p:pin,tip", "b:base"},
+			links: []Link{{Above: 3}, {Above: 2}},
+		},
+		{
+			name: "fork with the tip above the pin on the main lane",
+			st: Status{Pinned: c("p"), CheckedOut: c("y"), Tip: c("t"), Pin: PinInfo{Behind: 2},
+				Drift: Distance{Ahead: 1, Behind: 1}, Bases: Bases{Checkout: c("b")}},
+			nodes: []string{"y:you", "t:tip", "p:pin", "b:base"},
+			links: []Link{{Above: 1}, {Above: 2}, {Above: 1}},
+		},
+		{
+			name: "something older than the base continues below it",
+			st: Status{Pinned: c("p"), CheckedOut: c("y"), Tip: c("t"), Pin: PinInfo{Ahead: 1, Behind: 2},
+				Bases: Bases{Tip: c("b")}, Drift: Distance{Behind: 3}},
+			nodes: []string{"t:tip", "p:pin", "b:base", "y:you"},
+			links: []Link{{Above: 2}, {Above: 1}, {Above: 2}},
+		},
+		{
+			name: "the base is also the checkout",
+			st: Status{Pinned: c("p"), CheckedOut: c("b"), Tip: c("t"), Pin: PinInfo{Ahead: 1, Behind: 2},
+				Bases: Bases{Tip: c("b")}, Drift: Distance{Behind: 1}},
+			nodes: []string{"t:tip", "p:pin", "b:you,base"},
+			links: []Link{{Above: 2}, {Above: 1}},
+		},
+		{
+			name: "two diverged commits are not drawn as a fork",
+			st: Status{Pinned: c("p"), CheckedOut: c("y"), Tip: c("t"), Pin: PinInfo{Ahead: 1, Behind: 2},
+				Drift: Distance{Ahead: 1, Behind: 1}, Bases: Bases{Tip: c("b"), Checkout: c("b")}},
+			nodes: []string{"t:tip", "p:pin", "y:you"},
+			links: []Link{{Above: 2, Below: 1}, {Above: 1, Below: 1}},
+		},
 		{
 			name:  "no tip, checkout not loaded",
 			st:    Status{Pinned: c("p")},
@@ -92,7 +134,7 @@ func TestSpine(t *testing.T) {
 			links: []Link{{Above: 2}},
 		},
 	}
-	words := map[Role]string{RolePin: "pin", RoleStaged: "staged", RoleHead: "head", RoleTip: "tip", RoleYou: "you"}
+	words := map[Role]string{RolePin: "pin", RoleStaged: "staged", RoleHead: "head", RoleTip: "tip", RoleYou: "you", RoleBase: "base"}
 	for _, tt := range tests {
 		var nodes []string
 		var links []Link

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: "/",
 	}
 }