Support the mouse in the branch popup

A click on a branch switches to it, the wheel moves the selection, a click
outside closes the popup. The popup is hit-tested by its own position, as it
is drawn after zone.Scan.

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

internal/tui/branches/branches.go

@@ -35,6 +35,7 @@ type Model struct {
 	loading bool
 	err     error
 	spinner string
+	w, h    int // size of the popup as last rendered, for hit tests
 }
 
 // New returns a picker in the loading state.
@@ -177,11 +178,40 @@ func (m *Model) View(termWidth, termHeight int) string {
 	lines = append(lines, body[:m.rows]...)
 	lines = append(lines, muted.Render(ansi.Truncate("↑↓ select · enter switch · esc cancel", inner, "…")))
 
-	return lipgloss.NewStyle().
+	out := lipgloss.NewStyle().
 		Border(lipgloss.RoundedBorder()).
 		BorderForeground(m.theme.Border).
 		Padding(0, 1).
 		Render(strings.Join(lines, "\n"))
+	m.w, m.h = lipgloss.Width(out), lipgloss.Height(out)
+	return out
+}
+
+// Origin is the top-left cell of the popup, centered in a terminal of the
+// given size. Valid after View.
+func (m *Model) Origin(termWidth, termHeight int) (x, y int) {
+	return max((termWidth-m.w)/2, 0), max((termHeight-m.h)/2, 0)
+}
+
+// listTop is the line of the popup (border, title, search, rule) where the
+// list starts.
+const listTop = 4
+
+// Click handles a left click at cell (x, y) of a terminal of the given size.
+// inside reports whether the click was within the popup. If it hit a branch,
+// that branch is selected and hit is true.
+func (m *Model) Click(x, y, termWidth, termHeight int) (inside, hit bool) {
+	ox, oy := m.Origin(termWidth, termHeight)
+	x, y = x-ox, y-oy
+	if x < 0 || y < 0 || x >= m.w || y >= m.h {
+		return false, false
+	}
+	row := y - listTop
+	if m.loading || m.err != nil || x == 0 || x == m.w-1 || row < 0 || row >= m.rows || m.offset+row >= len(m.shown) {
+		return true, false
+	}
+	m.cursor = m.offset + row
+	return true, true
 }
 
 // row renders one branch: the name, right-aligned the short SHA and age.

internal/tui/branches/branches_test.go

@@ -127,3 +127,44 @@ func TestListScrollsInASmallTerminal(t *testing.T) {
 		t.Errorf("cursor %d outside the window [%d,%d)", m.cursor, m.offset, m.offset+m.rows)
 	}
 }
+
+func TestClickSelectsTheBranchUnderTheMouse(t *testing.T) {
+	m := New(theme.Default(), "src/app")
+	m.SetBranches(list())
+	m.View(100, 30)
+	x, y := m.Origin(100, 30)
+
+	if inside, hit := m.Click(x+3, y+listTop+1, 100, 30); !inside || !hit || name(m) != "dev" {
+		t.Errorf("click on the 2nd row: inside=%v hit=%v selected %q", inside, hit, name(m))
+	}
+	if inside, hit := m.Click(x+3, y+listTop+3, 100, 30); !inside || !hit || name(m) != "origin/feat/search" {
+		t.Errorf("click on the 4th row: inside=%v hit=%v selected %q", inside, hit, name(m))
+	}
+	for _, c := range []struct {
+		name string
+		x, y int
+	}{
+		{"title", x + 3, y + 1},
+		{"search line", x + 3, y + 2},
+		{"hint line", x + 3, y + m.h - 2},
+		{"left border", x, y + listTop},
+	} {
+		before := name(m)
+		if inside, hit := m.Click(c.x, c.y, 100, 30); !inside || hit || name(m) != before {
+			t.Errorf("click on the %s: inside=%v hit=%v selected %q", c.name, inside, hit, name(m))
+		}
+	}
+	if inside, hit := m.Click(0, 0, 100, 30); inside || hit {
+		t.Errorf("click outside: inside=%v hit=%v", inside, hit)
+	}
+}
+
+func TestClickBelowTheLastBranchHitsNothing(t *testing.T) {
+	m := New(theme.Default(), "x")
+	m.SetBranches(list()[:1]) // 1 branch, the list is padded to 3 rows
+	m.View(100, 30)
+	x, y := m.Origin(100, 30)
+	if inside, hit := m.Click(x+3, y+listTop+2, 100, 30); !inside || hit {
+		t.Errorf("inside=%v hit=%v on an empty row", inside, hit)
+	}
+}