Fix preview area not being cleared when using certain types of border styles

fzf --preview 'sleep 3; date' --preview-window hidden \
      --bind ctrl-/:change-preview-window:up,border-bottom

src/tui/light.go

@@ -808,13 +808,25 @@ func (w *LightWindow) drawBorderHorizontal(top, bottom bool) {
 		color = ColPreviewBorder
 	}
 	hw := runeWidth(w.border.top)
+	pad := repeat(' ', w.width/hw)
+
+	w.Move(0, 0)
 	if top {
-		w.Move(0, 0)
 		w.CPrint(color, repeat(w.border.top, w.width/hw))
+	} else {
+		w.CPrint(color, pad)
 	}
+
+	for y := 1; y < w.height-1; y++ {
+		w.Move(y, 0)
+		w.CPrint(color, pad)
+	}
+
+	w.Move(w.height-1, 0)
 	if bottom {
-		w.Move(w.height-1, 0)
 		w.CPrint(color, repeat(w.border.bottom, w.width/hw))
+	} else {
+		w.CPrint(color, pad)
 	}
 }
 

src/tui/tcell.go

@@ -544,7 +544,7 @@ func (r *FullscreenRenderer) NewWindow(top int, left int, width int, height int,
 		height:      height,
 		normal:      normal,
 		borderStyle: borderStyle}
-	w.drawBorder(false)
+	w.Erase()
 	return w
 }
 
@@ -561,8 +561,8 @@ func fill(x, y, w, h int, n ColorPair, r rune) {
 }
 
 func (w *TcellWindow) Erase() {
-	w.drawBorder(false)
 	fill(w.left-1, w.top, w.width+1, w.height-1, w.normal, ' ')
+	w.drawBorder(false)
 }
 
 func (w *TcellWindow) EraseMaybe() bool {