Cache Markdown text sizes per width to stop re-typesetting

mac/Sources/Byre/MarkdownView.swift

@@ -630,7 +630,8 @@ private final class FlowMeasure {
     private let storage = NSTextStorage()
     private let layout = NSLayoutManager()
     private let container = NSTextContainer()
-    private var measured: (text: NSAttributedString, width: CGFloat?, size: CGSize)?
+    private var text: NSAttributedString?
+    private var sizes: [CGFloat?: CGSize] = [:]
 
     init() {
         container.lineFragmentPadding = 0
@@ -638,19 +639,25 @@ private final class FlowMeasure {
         storage.addLayoutManager(layout)
     }
 
+    // SwiftUI asks each layout pass for several widths (unspecified, 0 and the real one), so one size per width.
     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 {
+        if let t = self.text, t === text || t.isEqual(to: text) {
+            if let size = sizes[width] {
+                return size
+            }
+        } else {
+            self.text = text
+            sizes = [:]
             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)
+        if sizes.count >= 8 {
+            sizes = [:]
+        }
+        sizes[width] = size
         return size
     }
 }