Stop the macOS transcript jumping while scrolling

mac/Sources/Byre/Chat.swift

@@ -495,6 +495,7 @@ private struct TranscriptScroll: View {
     @Environment(Store.self) private var store
     @Environment(\.paneAccent) private var accent
     @ViewState private var position = ScrollPosition(edge: .bottom)
+    @ViewState private var scrolling = false
 
     var body: some View {
         let running = model.running
@@ -536,13 +537,16 @@ private struct TranscriptScroll: View {
         .onScrollGeometryChange(for: ScrollMetrics.self) { g in
             ScrollMetrics(content: g.contentSize.height, visible: g.visibleRect.height, distance: g.contentSize.height - g.visibleRect.maxY)
         } action: { old, new in
-            if new.content != old.content || new.visible != old.visible {
-                if model.stuck && new.distance > 1 {
-                    position.scrollTo(edge: .bottom)
-                }
+            if scrolling {
+                model.stuck = new.distance < 80
                 return
             }
-            model.stuck = new.distance < 80
+            if model.stuck && new.distance > 1 && (new.content != old.content || new.visible != old.visible) {
+                position.scrollTo(edge: .bottom)
+            }
+        }
+        .onScrollPhaseChange { _, phase in
+            scrolling = phase.isScrolling
         }
         .onChange(of: model.builder.pending.count) {
             model.stuck = true

mac/Sources/Byre/MarkdownView.swift

@@ -425,16 +425,7 @@ private struct FlowText: NSViewRepresentable {
     }
 
     func sizeThatFits(_ proposal: ProposedViewSize, nsView v: FlowTextView, context: Context) -> CGSize? {
-        let width = proposal.width.flatMap { $0.isFinite ? $0 : nil }
-        let storage = NSTextStorage(attributedString: text)
-        let layout = NSLayoutManager()
-        let container = NSTextContainer(size: CGSize(width: width ?? .greatestFiniteMagnitude, height: .greatestFiniteMagnitude))
-        container.lineFragmentPadding = 0
-        layout.addTextContainer(container)
-        storage.addLayoutManager(layout)
-        layout.ensureLayout(for: container)
-        let used = layout.usedRect(for: container)
-        return CGSize(width: width ?? ceil(used.width), height: ceil(used.height))
+        context.coordinator.size(text, proposal.width.flatMap { $0.isFinite ? $0 : nil })
     }
 
     func makeCoordinator() -> Coordinator {
@@ -443,6 +434,33 @@ private struct FlowText: NSViewRepresentable {
 
     final class Coordinator: NSObject, NSTextViewDelegate {
         var openURL: OpenURLAction?
+        private let storage = NSTextStorage()
+        private let layout = NSLayoutManager()
+        private let container = NSTextContainer()
+        private var measured: (text: NSAttributedString, width: CGFloat?, size: CGSize)?
+
+        override init() {
+            super.init()
+            container.lineFragmentPadding = 0
+            layout.addTextContainer(container)
+            storage.addLayoutManager(layout)
+        }
+
+        func size(_ text: NSAttributedString, _ width: CGFloat?) -> CGSize {
+            let same = measured.map { $0.text === text || $0.text.isEqual(to: text) } ?? false
+            if same, let m = measured, m.width == width {
+                return m.size
+            }
+            if !same {
+                storage.setAttributedString(text)
+            }
+            container.size = CGSize(width: width ?? .greatestFiniteMagnitude, height: .greatestFiniteMagnitude)
+            layout.ensureLayout(for: container)
+            let used = layout.usedRect(for: container)
+            let size = CGSize(width: width ?? ceil(used.width), height: ceil(used.height))
+            measured = (text, width, size)
+            return size
+        }
 
         func textView(_ textView: NSTextView, clickedOnLink link: Any, at charIndex: Int) -> Bool {
             guard let url = link as? URL, let openURL else {