Fix rendering of preview window border of tcell renderer

(sleep 1; find .) |
    go run -tags tcell main.go --bind 'space:change-preview-window(60%|70%|80%|90%|border-left|border-right|border-vertical|border-top|border-horizontal|border-bottom|border-sharp|border-double|border-block|hidden|left|up|down|right|up|down|)' \
        --preview 'cat {}' --color bg:red,preview-bg:blue \
        --border --margin 3

src/tui/tcell.go

@@ -561,7 +561,11 @@ func fill(x, y, w, h int, n ColorPair, r rune) {
 }
 
 func (w *TcellWindow) Erase() {
-	fill(w.left-1, w.top, w.width+1, w.height-1, w.normal, ' ')
+	if w.borderStyle.shape.HasLeft() {
+		fill(w.left-1, w.top, w.width, w.height-1, w.normal, ' ')
+	} else {
+		fill(w.left, w.top, w.width-1, w.height-1, w.normal, ' ')
+	}
 	w.drawBorder(false)
 }
 

src/tui/tui.go

@@ -368,6 +368,14 @@ const (
 	BorderRight
 )
 
+func (s BorderShape) HasLeft() bool {
+	switch s {
+	case BorderNone, BorderRight, BorderTop, BorderBottom, BorderHorizontal: // No Left
+		return false
+	}
+	return true
+}
+
 func (s BorderShape) HasRight() bool {
 	switch s {
 	case BorderNone, BorderLeft, BorderTop, BorderBottom, BorderHorizontal: // No right