TextLayoutManager: Reduce viewport top inset

The STTextView's viewport inset is reduced from half the visible height to a
quarter of the visible height. This improves performance by preparing a
smaller area of content ahead of the currently visible region.

Author: krzyzanowskim

Sources/STTextViewAppKit/STTextView.swift

@@ -824,7 +824,7 @@ import AVFoundation
     }
 
     open override func prepareContent(in rect: NSRect) {
-        var insetRect = rect.inset(dy: -visibleRect.height / 2)
+        var insetRect = rect.inset(dy: -visibleRect.height / 4)
         insetRect.origin = CGPoint(x: max(0, insetRect.origin.x), y: max(insetRect.origin.y, 0))
         super.prepareContent(in: insetRect)
         layoutViewport()