zero-swift: a list stuck to the bottom takes the user's first scroll

.changeset/stuck-list-first-scroll.md

@@ -0,0 +1,5 @@
+---
+'zero-swift': patch
+---
+
+A `VirtualList` or `VirtualListView` stuck to the bottom (`stickToBottom`) no longer pulls the user's first scroll up back to the bottom. When the list tried to scroll past where it could, its next scroll, the user's, was taken for its own, and the list stayed stuck.

swift/Sources/ZeroVirtual/VirtualizerCore.swift

@@ -719,10 +719,17 @@ package final class VirtualizerCore<
   @discardableResult
   private func setScrollOffset(_ offset: Double) -> Bool {
     guard let host, host.scrollOffset.rounded() != offset.rounded() else { return false }
+    let from = host.scrollOffset
     programmaticScroll = true
     writing = true
     host.scroll(to: offset)
     writing = false
+    guard host.scrollOffset != from else {
+      // Clamped to where it was: no scroll, and the user's next one isn't
+      // ours.
+      programmaticScroll = false
+      return false
+    }
     if depth == 0, scrollDuringWrite {
       scrollDuringWrite = false
       onScroll()

swift/Tests/ZeroVirtualTests/LayoutHostTests.swift

@@ -269,6 +269,27 @@ final class LayoutScrollView {
     #expect(view.scrollOffset == offset)
   }
 
+  @Test("stuck to the bottom of a list opened at its last row, a scroll up moves the rows by it")
+  func scrollUpFromTheBottom() {
+    let h = Harness(rowCount: 400, rowHeight: 80) {
+      $0.anchoring = .manual
+      $0.permalinkID = "r399"
+      $0.stickToBottom = true
+      $0.minPageSize = 20
+    }
+    let view = LayoutScrollView(h)
+    for i in 0..<400 { view.heights["r\(i)"] = [62.3, 180.6, 38.5, 121.0, 76.0][i % 5] }
+    view.settle()
+    h.scheduler.advance(by: 200)
+    view.settle()
+    #expect(view.bottomGap == 0)
+    // The core's scrolls past the bottom stay where they are: the user's
+    // next scroll is the user's, not taken back to the bottom.
+    let top = try! #require(view.rowTop("r396"))
+    view.userScroll(view.scrollOffset - 20.5)
+    #expect(view.rowTop("r396") == top + 20.5)
+  }
+
   @Test("scrolls to a row not loaded yet")
   func scrollToItemFarBelow() {
     let h = Harness(rowCount: 200) { $0.anchoring = .manual }