feat: show grab line when sidebar is collapsed

Allow dragging the sidebar back open by clicking and dragging
the thin vertical line that remains when sidebar is hidden.

Author: pablospe

pkg/ui/mainModel.go

@@ -32,6 +32,9 @@ const (
 	zoneFileTree      = "filetree"
 	zoneSearchResults = "searchresults"
 	zoneDiffViewer    = "diffviewer"
+
+	// Sidebar resize detection threshold in pixels.
+	sidebarGrabThreshold = 2
 )
 
 type mainModel struct {
@@ -260,6 +263,15 @@ func (m mainModel) View() string {
 			Width(width).
 			Border(lipgloss.NormalBorder(), false, true, false, false).
 			BorderForeground(lipgloss.Color("8")).Render(content)
+	} else {
+		// Show a thin grab line when sidebar is hidden.
+		grabLine := lipgloss.NewStyle().
+			Width(0).
+			Height(m.height - footerHeight - headerHeight).
+			Border(lipgloss.NormalBorder(), false, true, false, false).
+			BorderForeground(lipgloss.Color("8")).
+			Render("")
+		sidebar = grabLine
 	}
 	dv := lipgloss.NewStyle().MaxHeight(m.height - m.footerHeight() - m.headerHeight()).Width(m.width - m.sidebarWidth()).Render(m.diffViewer.View())
 	dv = zone.Mark(zoneDiffViewer, dv)
@@ -377,8 +389,14 @@ func (m mainModel) handleMouse(msg tea.MouseMsg) (tea.Model, tea.Cmd) {
 		if msg.Button == tea.MouseButtonLeft {
 			// Keep coordinate check for resize border (hybrid approach).
 			sidebarWidth := m.sidebarWidth()
-			if m.isShowingFileTree && abs(msg.X-sidebarWidth) <= 2 {
+			if m.isShowingFileTree && abs(msg.X-sidebarWidth) <= sidebarGrabThreshold {
+				m.draggingSidebar = true
+				return m, nil
+			}
+			// Allow grabbing the line when sidebar is hidden.
+			if !m.isShowingFileTree && msg.X <= sidebarGrabThreshold {
 				m.draggingSidebar = true
+				m.isShowingFileTree = true
 				return m, nil
 			}