Real-image posters: draw placeholder cells on the panes' background color

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

internal/ui/kitty_test.go

@@ -12,6 +12,7 @@ import (
 	"testing"
 
 	"github.com/gdamore/tcell/v2"
+	"github.com/rivo/tview"
 )
 
 // recorder collects what is written to the terminal.
@@ -138,10 +139,13 @@ func TestPlaceholderCell(t *testing.T) {
 	if ch != '\U0010EEEE' || len(comb) != 2 || comb[0] != kittyDiacritics[2] || comb[1] != kittyDiacritics[5] {
 		t.Errorf("cell = %U %U", ch, comb)
 	}
-	fg, _, _ := style.Decompose()
+	fg, bg, _ := style.Decompose()
 	if r, g, b := fg.RGB(); r != 1 || g != 2 || b != 3 {
 		t.Errorf("the id must be in the foreground color, got %d,%d,%d", r, g, b)
 	}
+	if bg != tview.Styles.PrimitiveBackgroundColor {
+		t.Errorf("background %v must match the panes (%v), or uncovered slivers show", bg, tview.Styles.PrimitiveBackgroundColor)
+	}
 }
 
 func TestKittyTerminalDetection(t *testing.T) {