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/overlay.go

@@ -7,28 +7,27 @@ import (
 	"github.com/charmbracelet/x/ansi"
 )
 
-// overlay draws fg centered over bg, which is w x h cells. bg must be the
+// overlay draws fg over bg with its top-left cell at (x, y). bg must be the
 // final string (after zone.Scan): its lines are cut at the popup's edges.
 // The backdrop loses its colors and is drawn in dim, so the popup stands out.
-func overlay(bg, fg string, w, h int, dim lipgloss.Style) string {
+func overlay(bg, fg string, x, y int, dim lipgloss.Style) string {
 	lines := strings.Split(bg, "\n")
 	for i, l := range lines {
 		lines[i] = dim.Render(ansi.Strip(l))
 	}
 	pop := strings.Split(fg, "\n")
 	pw := lipgloss.Width(fg)
-	top := max((h-len(pop))/2, 0)
-	left := max((w-pw)/2, 0)
+	left := x
 	for i, p := range pop {
-		y := top + i
-		if y >= len(lines) {
+		row := y + i
+		if row >= len(lines) {
 			break
 		}
-		line := lines[y]
+		line := lines[row]
 		if lw := ansi.StringWidth(line); lw < left {
 			line += strings.Repeat(" ", left-lw)
 		}
-		lines[y] = ansi.Truncate(line, left, "") + "\x1b[0m" + p + "\x1b[0m" + ansi.TruncateLeft(line, left+pw, "")
+		lines[row] = ansi.Truncate(line, left, "") + "\x1b[0m" + p + "\x1b[0m" + ansi.TruncateLeft(line, left+pw, "")
 	}
 	return strings.Join(lines, "\n")
 }