fix: round corners would be aliased after changing theme (#1698)

src/ui/main-window/ThumbnailsView.swift

@@ -19,15 +19,20 @@ class ThumbnailsView: NSVisualEffectView {
     /// using layer!.cornerRadius works but the corners are aliased; this custom approach gives smooth rounded corners
     /// see https://stackoverflow.com/a/29386935/2249756
     func updateRoundedCorners(_ cornerRadius: CGFloat) {
-        let edgeLength = 2.0 * cornerRadius + 1.0
-        maskImage = NSImage(size: NSSize(width: edgeLength, height: edgeLength), flipped: false) { rect in
-            let bezierPath = NSBezierPath(roundedRect: rect, xRadius: cornerRadius, yRadius: cornerRadius)
-            NSColor.black.set()
-            bezierPath.fill()
-            return true
+        if cornerRadius == 0 {
+            maskImage = nil
+        } else {
+            let edgeLength = 2.0 * cornerRadius + 1.0
+            let mask = NSImage(size: NSSize(width: edgeLength, height: edgeLength), flipped: false) { rect in
+                let bezierPath = NSBezierPath(roundedRect: rect, xRadius: cornerRadius, yRadius: cornerRadius)
+                NSColor.black.set()
+                bezierPath.fill()
+                return true
+            }
+            mask.capInsets = NSEdgeInsets(top: cornerRadius, left: cornerRadius, bottom: cornerRadius, right: cornerRadius)
+            mask.resizingMode = .stretch
+            maskImage = mask
         }
-        maskImage!.capInsets = NSEdgeInsets(top: cornerRadius, left: cornerRadius, bottom: cornerRadius, right: cornerRadius)
-        maskImage!.resizingMode = .stretch
     }
 
     func nextRow(_ direction: Direction) -> [ThumbnailView]? {