fix: scrollbar only shows on scroll (closes #196)

src/ui/main-window/ThumbnailsPanel.swift

@@ -22,6 +22,7 @@ class ThumbnailsPanel: NSPanel {
 
     func show() {
         makeKeyAndOrderFront(nil)
+        thumbnailsView.scrollView.flashScrollers()
     }
 
     static func highlightCell(_ previousView: NSView, _ newView: NSView) {

src/ui/main-window/ThumbnailsView.swift

@@ -92,11 +92,13 @@ class ScrollView: NSScrollView {
         documentView = FlippedView(frame: .zero)
         drawsBackground = false
         hasVerticalScroller = true
+        scrollerStyle = .overlay
+        scrollerKnobStyle = .light
     }
 
+    // draw the scroller on top of the content
     override func tile() {
         super.tile()
-        // draw the scroller on top of the content
         contentView.frame = bounds
     }
 }