feat(D.2b): UiScrollable — pixel scroll model (UIElement_Scrollable port)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/AcDream.App/UI/UiScrollable.cs

@@ -0,0 +1,57 @@
+using System;
+
+namespace AcDream.App.UI;
+
+/// <summary>
+/// Pixel-based vertical scroll model. Port of retail <c>UIElement_Scrollable</c>:
+/// the scroll offset is an integer pixel value (<c>m_iScrollableY</c>) clamped to
+/// [0, ContentHeight - ViewHeight]; the thumb ratio is view/content; the position
+/// ratio is scroll/(content-view). Pure (no GL) so it is fully unit-tested and
+/// shared by the transcript (UiChatView) and the scrollbar (UiChatScrollbar).
+/// Decomp anchors: SetScrollableXY @0x4740c0, UpdateScrollbarSize_ @0x4741a0,
+/// UpdateScrollbarPosition_ @0x473f20, UIElement_Text::InqScrollDelta @0x4689b0.
+/// </summary>
+public sealed class UiScrollable
+{
+    /// <summary>Total wrapped content height in px (m_iScrollableHeight).</summary>
+    public int ContentHeight { get; set; }
+    /// <summary>Visible viewport height in px.</summary>
+    public int ViewHeight { get; set; }
+    /// <summary>Pixels per text line (scroll quantum). InqScrollDelta line case.</summary>
+    public int LineHeight { get; set; } = 16;
+
+    private int _scrollY;
+    /// <summary>Current scroll offset in px from the top of the content.</summary>
+    public int ScrollY => _scrollY;
+
+    /// <summary>Max scroll = max(0, content - view).</summary>
+    public int MaxScroll => Math.Max(0, ContentHeight - ViewHeight);
+
+    /// <summary>True when content exceeds the view (a scrollbar is warranted).</summary>
+    public bool HasOverflow => ContentHeight > ViewHeight;
+
+    /// <summary>True when the offset is at (or past) the bottom — used for bottom-pin.</summary>
+    public bool AtEnd => _scrollY >= MaxScroll;
+
+    /// <summary>Set the offset, clamped to [0, MaxScroll] (SetScrollableXY clamp).</summary>
+    public void SetScrollY(int y) => _scrollY = Math.Clamp(y, 0, MaxScroll);
+
+    /// <summary>Pin to the bottom (newest content visible).</summary>
+    public void ScrollToEnd() => _scrollY = MaxScroll;
+
+    /// <summary>Thumb size ratio = view/content, clamped to 1 (UpdateScrollbarSize_).</summary>
+    public float ThumbRatio => ContentHeight <= 0 ? 1f : Math.Min(1f, (float)ViewHeight / ContentHeight);
+
+    /// <summary>Position ratio = scroll/(content-view) in [0,1] (UpdateScrollbarPosition_).</summary>
+    public float PositionRatio => MaxScroll <= 0 ? 0f : (float)_scrollY / MaxScroll;
+
+    /// <summary>Inverse of PositionRatio — used when the user drags the thumb.</summary>
+    public void SetPositionRatio(float ratio)
+        => SetScrollY((int)MathF.Round(Math.Clamp(ratio, 0f, 1f) * MaxScroll));
+
+    /// <summary>Scroll by whole lines (sign: +down/newer, -up/older).</summary>
+    public void ScrollByLines(int lines) => SetScrollY(_scrollY + lines * LineHeight);
+
+    /// <summary>Scroll by a page = one view height (InqScrollDelta page case).</summary>
+    public void ScrollByPage(int pages) => SetScrollY(_scrollY + pages * ViewHeight);
+}