ios: isolate Agent input and read observation

ios/Tests/SybilUITests/TranscriptFadeUITests.swift

@@ -37,8 +37,8 @@ final class TranscriptFadeUITests: XCTestCase {
     }
 
     private var transcript: XCUIElement {
-        // The tablet sidebar is also a scroll view; select the detail by its content.
-        app.scrollViews.containing(.staticText, identifier: "History message 160: local simulator fixture.").firstMatch
+        // Identify the transcript independently of any particular message row.
+        app.scrollViews["agent-transcript"]
     }
 
     private func capture(_ name: String) {
@@ -107,7 +107,7 @@ final class TranscriptFadeUITests: XCTestCase {
         capture("agent-fade-reading-position-and-unread")
 
         for _ in 0..<30 {
-            if app.buttons["Load earlier messages"].isHittable { break }
+            if app.buttons["Load earlier messages"].exists && app.buttons["Load earlier messages"].isHittable { break }
             scroll.swipeDown(velocity: .fast)
         }
         let load = app.buttons["Load earlier messages"]