Fix drop target overlay (#376)

Author: microbit-grace

src/components/FileDropTarget.tsx

@@ -49,7 +49,7 @@ const FileDropTarget = ({ children, onFileDrop }: FileDropTargetProps) => {
         <Center
           onDrop={handleDrop}
           onDragLeave={handleDragLeave}
-          position="absolute"
+          position="fixed"
           top={0}
           left={0}
           height="100%"