osx: better scroll view management

osx/kordophone2/Transcript/TranscriptDisplayItems.swift

@@ -10,7 +10,7 @@ import SwiftUI
 
 extension TranscriptView.ViewModel
 {
-    internal func rebuildDisplayItems(animated: Bool = false) {
+    internal func rebuildDisplayItems(animated: Bool = false, completion: () -> Void = {}) {
         var displayItems: [DisplayItem] = []
         var lastDate: Date = .distantPast
         var lastSender: Display.Sender? = nil
@@ -53,6 +53,7 @@ extension TranscriptView.ViewModel
         let animation: Animation? = animated ? .default : nil
         withAnimation(animation) {
             self.displayItems = displayItems
+            completion()
         }
     }
 }