On macOS 11 set the outline view style to NSTableViewStylePlain.

This eliminates the padding added to the outline view on Big Sur.

Use KVC to allow compiling without the macOS 11 SDK.
The method -[NSTableView setStyle:] was not available before then.

source/BrowserWindowController.swift

@@ -66,6 +66,9 @@ class BrowserWindowController:NSObject, NSOutlineViewDataSource, NSOutlineViewDe
 		outlineView.headerView = nil
 		outlineView.intercellSpacing = NSMakeSize(3.0, 2.0) // macOS 11 SDK changed the default, so use the old value.
 		outlineView.rowHeight = 17.0 // macOS 11 SDK changed the default, so use the old value.
+		if #available(macOS 11.0, *) {
+			outlineView.setValue(4, forKey:"style") // NSTableViewStylePlain
+		}
 		outlineView.setDraggingSourceOperationMask([.copy], forLocal:false) // Allow drags to other apps
 		outlineView.usesAlternatingRowBackgroundColors = false