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

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

internal/ui/kitty.go

@@ -9,6 +9,7 @@ import (
 	"strings"
 
 	"github.com/gdamore/tcell/v2"
+	"github.com/rivo/tview"
 )
 
 // The Kitty graphics protocol with Unicode placeholders, as supported by
@@ -139,7 +140,9 @@ func (k *kittyGraphics) deleteAll() {
 func placeholderCell(id uint32, row, col int) (main rune, comb []rune, style tcell.Style) {
 	comb = []rune{kittyDiacritics[row], kittyDiacritics[col]}
 	fg := tcell.NewRGBColor(int32(id>>16&0xFF), int32(id>>8&0xFF), int32(id&0xFF))
-	return placeholderRune, comb, tcell.StyleDefault.Foreground(fg)
+	// The background shows wherever the picture does not cover the cell (a
+	// sliver at the edge): use the panes' own color so it is not visible.
+	return placeholderRune, comb, tcell.StyleDefault.Foreground(fg).Background(tview.Styles.PrimitiveBackgroundColor)
 }
 
 // maxPlaceholders is how many rows and columns can be numbered.