Fix stray character artifacts when scrollbar is hidden

Fix #4537

src/terminal.go

@@ -2397,6 +2397,13 @@ func (t *Terminal) resizeWindows(forcePreview bool, redrawBorder bool) {
 			innerHeight-shrink, tui.WindowList, noBorder, true)
 	}
 
+	if len(t.scrollbar) == 0 {
+		for y := 0; y < t.window.Height(); y++ {
+			t.window.Move(y, t.window.Width()-1)
+			t.window.Print(" ")
+		}
+	}
+
 	createInnerWindow := func(b tui.Window, shape tui.BorderShape, windowType tui.WindowType, shift int) tui.Window {
 		top := b.Top()
 		left := b.Left() + shift

test/test_layout.rb

@@ -1192,6 +1192,29 @@ class TestLayout < TestInteractive
     tmux.until { assert_block(block, it) }
   end
 
+  # https://github.com/junegunn/fzf/issues/4537
+  def test_no_scrollbar_preview_toggle
+    x = 'x' * 300
+    y = 'y' * 300
+    tmux.send_keys %(yes #{x} | head -1000 | fzf --bind 'tab:toggle-preview' --border --no-scrollbar --preview 'echo #{y}' --preview-window 'border-left'), :Enter
+
+    # │ ▌ xxxxxxxx·· │ yyyyyyyy│
+    tmux.until do |lines|
+      lines.any? { it.match?(/x·· │ y+│$/) }
+    end
+    tmux.send_keys :Tab
+
+    # │ ▌ xxxxxxxx·· │
+    tmux.until do |lines|
+      lines.none? { it.match?(/x··y│$/) }
+    end
+
+    tmux.send_keys :Tab
+    tmux.until do |lines|
+      lines.any? { it.match?(/x·· │ y+│$/) }
+    end
+  end
+
   def test_combinations
     skip unless ENV['LONGTEST']