fix(scroll): keep mouse wheel / trackball scrollable past a stale-small clamp

Sources/MarkdownEngine/TextView/ClampedScrollView.swift

@@ -38,10 +38,24 @@ final class ClampedScrollView: NSScrollView {
         // Use the real content height (not the inflated frame) so small
         // documents can't scroll past their actual content. The document view is
         // always a `NativeTextViewContainer` (header band + text column).
-        let realHeight = (doc as? NativeTextViewContainer)?.scrollableContentHeight
-            ?? doc.bounds.height
-        let maxY = max(minY, realHeight - contentView.bounds.height)
+        let container = doc as? NativeTextViewContainer
+        var realHeight = container?.scrollableContentHeight ?? doc.bounds.height
         let b = contentView.bounds
+
+        // `scrollableContentHeight` comes from a cached TextKit-2 measurement that can
+        // under-measure. A continuous trackpad refreshes it mid-gesture, but a discrete
+        // device (mouse wheel, Ploopy trackball) sends one event with no relayout — so a
+        // stale-small height clamps the tick straight back = "scroll doesn't work". When
+        // a clamp-back is imminent, force one fresh full-layout re-measure first. This is
+        // self-limiting (only at the bottom) and still clamps to the real content height.
+        if let textView = container?.textView,
+           b.origin.y > realHeight - b.height {
+            textView.pendingFullLayoutMeasure = true
+            textView.recalcOverscroll(for: self)
+            realHeight = container?.scrollableContentHeight ?? doc.bounds.height
+        }
+
+        let maxY = max(minY, realHeight - contentView.bounds.height)
         let clampedY = min(max(b.origin.y, minY), maxY)
         if clampedY != b.origin.y {
             contentView.scroll(to: NSPoint(x: b.origin.x, y: clampedY))