feat(D.2b): chat text selection + Ctrl-C copy

Windows-like selection in the retail chat window: left-click-drag selects
characters, Ctrl-C copies, Ctrl-A selects all. The selected span paints a
translucent highlight behind the text.

- UiElement.CapturesPointerDrag: a per-element opt-out so an interior drag is
  delivered to the widget (text selection) instead of moving/resizing the host
  window. UiRoot.OnMouseDown honours it AFTER edge-resize (a resizable window
  is still resizable from its frame) and BEFORE window-move.
- UiChatView: AcceptsFocus + IsEditControl + CapturesPointerDrag; caches the
  OnDraw layout so OnEvent hit-tests the same geometry; HitChar maps a local
  point to (line,col) with glyph-midpoint caret snapping; SelectedText joins a
  multi-line span with \n; Ctrl-C writes to IKeyboard.ClipboardText (only when
  non-empty, so an empty copy never clobbers the clipboard).
- UiHost exposes the wired IKeyboard (clipboard + Ctrl modifier state).

Adversarial-review fix (the 99 tests would have stayed green without it): a
coordinate-frame mismatch between MouseDown and MouseMove. UiRoot.OnMouseDown
dispatched HitTestTopDown's coords, which are relative to the TOP-LEVEL child,
while MouseMove/MouseUp use target.ScreenPosition. For the chat view inset at
(8,8) inside its window the anchor landed ~8px off the click. OnMouseDown now
delivers target-LOCAL coords like the other mouse events. Added a UiRoot
regression test asserting MouseDown and MouseMove share the target-local frame
for a nested child.

Decomp ref: SurfaceWindow text/selection model; clipboard via Silk.NET
IKeyboard.ClipboardText. Built with the chat-select-copy implement->review
workflow.

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

src/AcDream.App/Rendering/GameWindow.cs

@@ -1812,6 +1812,9 @@ public sealed class GameWindow : IDisposable
                         | AcDream.App.UI.AnchorEdges.Right | AcDream.App.UI.AnchorEdges.Bottom,
                 Font = _debugFont,
                 LinesProvider = () => BuildRetailChatLines(retailChatVm),
+                // Drag-select + Ctrl+C copy need the keyboard for clipboard +
+                // modifier state. UiHost.Keyboard is set during WireKeyboard above.
+                Keyboard = _uiHost.Keyboard,
             };
             chatWindow.AddChild(chatView);
             _uiHost.Root.AddChild(chatWindow);

src/AcDream.App/UI/UiChatView.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Numerics;
+using System.Text;
 using AcDream.App.Rendering;
 
 namespace AcDream.App.UI;
@@ -12,10 +13,10 @@ namespace AcDream.App.UI;
 /// text inside the window.
 ///
 /// <para>
-/// This is the read-only foundation. A follow-up sub-step adds glScissor-based
+/// Supports Windows-like text selection: a left-click-drag inside the transcript
-/// clipping + word-wrap, drag-to-select, and Ctrl+C copy (which needs the
+/// selects characters (the <see cref="UiElement.CapturesPointerDrag"/> opt-out
-/// <see cref="UiElement.CapturesPointerDrag"/> opt-out so an interior drag
+/// stops that interior drag from moving the host window), and Ctrl+C copies the
-/// selects text instead of moving the window).
+/// selected span to the clipboard. Ctrl+A selects everything.
 /// </para>
 /// </summary>
 public sealed class UiChatView : UiElement
@@ -23,15 +24,26 @@ public sealed class UiChatView : UiElement
     /// <summary>One display line: pre-formatted text + its colour.</summary>
     public readonly record struct Line(string Text, Vector4 Color);
 
+    /// <summary>A caret position: a line index into the cached line list plus a
+    /// character index (0..line.Text.Length, i.e. a caret slot between glyphs).</summary>
+    public readonly record struct Pos(int Line, int Col);
+
     /// <summary>Provider of the lines to show, oldest-first. Polled each frame.</summary>
     public Func<IReadOnlyList<Line>> LinesProvider { get; set; } = static () => Array.Empty<Line>();
 
     /// <summary>Font for the transcript; falls back to the context default.</summary>
     public BitmapFont? Font { get; set; }
 
+    /// <summary>Keyboard device for clipboard (Ctrl+C) + modifier state. Wired by
+    /// the host from <see cref="UiHost.Keyboard"/>.</summary>
+    public Silk.NET.Input.IKeyboard? Keyboard { get; set; }
+
     /// <summary>Backing fill behind the text (retail chat is a dark translucent box).</summary>
     public Vector4 BackgroundColor { get; set; } = new(0f, 0f, 0f, 0.35f);
 
+    /// <summary>Highlight colour painted behind a selected character span.</summary>
+    public Vector4 SelectionColor { get; set; } = new(0.25f, 0.45f, 0.85f, 0.5f);
+
     /// <summary>Inner text inset from the view edges, px.</summary>
     public float Padding { get; set; } = 4f;
 
@@ -39,6 +51,25 @@ public sealed class UiChatView : UiElement
     private float _scroll;
     private const float WheelLines = 3f;   // lines advanced per wheel notch
 
+    // ── Cached layout from the last OnDraw, so OnEvent hit-tests the SAME geometry ──
+    private IReadOnlyList<Line> _lastLines = Array.Empty<Line>();
+    private BitmapFont? _lastFont;
+    private float _lastLineHeight = 16f;
+    private float _lastBaseY;          // top Y of line 0 in local space
+    private float _lastPadding = 4f;
+
+    // ── Selection state ──────────────────────────────────────────────────
+    private Pos? _selAnchor;   // where the drag started
+    private Pos? _selCaret;    // where the drag currently is
+    private bool _selecting;
+
+    public UiChatView()
+    {
+        AcceptsFocus = true;
+        IsEditControl = true;          // absorb keys (Ctrl+C) while focused
+        CapturesPointerDrag = true;    // interior drag selects, doesn't move the window
+    }
+
     /// <summary>
     /// Clamp a scroll offset to [0, max] where max = content-height - view-height
     /// (never negative — when everything fits, scroll is pinned to 0). Exposed for tests.
@@ -58,6 +89,14 @@ public sealed class UiChatView : UiElement
         if (font is null) return;
 
         var lines = LinesProvider();
+
+        // Cache the geometry OnEvent will hit-test against. Even when there are no
+        // lines we record the font/padding so a stray hit-test is harmless.
+        _lastLines = lines;
+        _lastFont = font;
+        _lastLineHeight = font.LineHeight;
+        _lastPadding = Padding;
+
         if (lines.Count == 0) return;
 
         float lh = font.LineHeight;
@@ -69,23 +108,244 @@ public sealed class UiChatView : UiElement
         // Bottom-pin: with _scroll==0 the LAST line ends at `bottom`; scrolling up
         // shifts the whole block down so older lines are revealed at the top.
         float baseY = bottom - contentH + _scroll;
+        _lastBaseY = baseY;
+
+        // Normalised selection span (start <= end), if any.
+        bool hasSel = TryGetOrderedSelection(out Pos selStart, out Pos selEnd);
+
         for (int i = 0; i < lines.Count; i++)
         {
             float y = baseY + i * lh;
             if (y < top || y + lh > bottom) continue;   // whole-line vertical clip (no scissor yet)
-            ctx.DrawString(lines[i].Text, Padding, y, lines[i].Color, font);
+
+            string text = lines[i].Text;
+
+            // Selection highlight behind this line's selected character span.
+            if (hasSel && i >= selStart.Line && i <= selEnd.Line)
+            {
+                int c0 = i == selStart.Line ? selStart.Col : 0;
+                int c1 = i == selEnd.Line ? selEnd.Col : text.Length;
+                c0 = Math.Clamp(c0, 0, text.Length);
+                c1 = Math.Clamp(c1, 0, text.Length);
+                if (c1 > c0)
+                {
+                    float hx = Padding + font.MeasureWidth(text.Substring(0, c0));
+                    float hw = font.MeasureWidth(text.Substring(c0, c1 - c0));
+                    ctx.DrawRect(hx, y, hw, lh, SelectionColor);
+                }
+            }
+
+            ctx.DrawString(text, Padding, y, lines[i].Color, font);
         }
     }
 
     public override bool OnEvent(in UiEvent e)
     {
-        if (e.Type == UiEventType.Scroll)
+        switch (e.Type)
         {
-            float lh = Font?.LineHeight ?? 16f;
+            case UiEventType.Scroll:
+            {
+                float lh = (Font ?? _lastFont)?.LineHeight ?? 16f;
                 // Silk wheel +Y = scroll up = reveal older = shift content down = larger _scroll.
                 _scroll += e.Data0 * WheelLines * lh;   // re-clamped next OnDraw against live content
                 return true;
             }
+
+            case UiEventType.MouseDown:
+            {
+                // Data1/Data2 = local-to-target coords (UiRoot.OnMouseDown).
+                var p = HitChar(e.Data1, e.Data2);
+                _selAnchor = p;
+                _selCaret = p;
+                _selecting = true;
+                return true;
+            }
+
+            case UiEventType.MouseMove:
+            {
+                if (_selecting)
+                {
+                    // Data1/Data2 = local-to-target coords (DispatchMouseMove).
+                    _selCaret = HitChar(e.Data1, e.Data2);
+                    return true;
+                }
                 return false;
             }
+
+            case UiEventType.MouseUp:
+            {
+                _selecting = false;
+                return true;
+            }
+
+            case UiEventType.KeyDown:
+            {
+                var key = (Silk.NET.Input.Key)e.Data0;
+                bool ctrl = Keyboard is not null
+                    && (Keyboard.IsKeyPressed(Silk.NET.Input.Key.ControlLeft)
+                        || Keyboard.IsKeyPressed(Silk.NET.Input.Key.ControlRight));
+                if (ctrl && key == Silk.NET.Input.Key.C)
+                {
+                    // Only touch the clipboard when there's a selection — an empty
+                    // copy must NOT clobber what the user previously copied.
+                    if (Keyboard is not null)
+                    {
+                        string sel = SelectedText();
+                        if (sel.Length > 0) Keyboard.ClipboardText = sel;
+                    }
+                    return true;
+                }
+                if (ctrl && key == Silk.NET.Input.Key.A)
+                {
+                    SelectAll();
+                    return true;
+                }
+                return false;
+            }
+        }
+        return false;
+    }
+
+    // ── Selection helpers ────────────────────────────────────────────────
+
+    /// <summary>Select the entire cached transcript (Ctrl+A).</summary>
+    private void SelectAll()
+    {
+        var lines = _lastLines;
+        if (lines.Count == 0)
+        {
+            _selAnchor = _selCaret = null;
+            return;
+        }
+        int last = lines.Count - 1;
+        _selAnchor = new Pos(0, 0);
+        _selCaret = new Pos(last, lines[last].Text.Length);
+    }
+
+    /// <summary>Normalise (anchor, caret) into ordered (start, end). False if no
+    /// selection or it is empty (anchor == caret).</summary>
+    private bool TryGetOrderedSelection(out Pos start, out Pos end)
+    {
+        start = default; end = default;
+        if (_selAnchor is not { } a || _selCaret is not { } c) return false;
+        (start, end) = Order(a, c);
+        return !(start.Line == end.Line && start.Col == end.Col);
+    }
+
+    /// <summary>The currently-selected text against the cached lines. Empty when
+    /// nothing is selected.</summary>
+    public string SelectedText()
+    {
+        if (!TryGetOrderedSelection(out var start, out var end)) return string.Empty;
+        return SelectedText(_lastLines, start, end);
+    }
+
+    // ── Pure, testable logic (no GL / no font texture) ───────────────────
+
+    /// <summary>Order two caret positions so the first is <= the second (by line,
+    /// then column).</summary>
+    public static (Pos start, Pos end) Order(Pos a, Pos b)
+    {
+        if (a.Line < b.Line || (a.Line == b.Line && a.Col <= b.Col)) return (a, b);
+        return (b, a);
+    }
+
+    /// <summary>
+    /// Assemble the selected substring spanning <paramref name="start"/> ..
+    /// <paramref name="end"/> (inclusive of start.Col, exclusive of end.Col) from
+    /// <paramref name="lines"/>. Multi-line selections are joined with "\n":
+    /// the first line from start.Col to its end, whole middle lines, and the last
+    /// line up to end.Col. Pure — unit-testable without GL.
+    /// </summary>
+    public static string SelectedText(IReadOnlyList<Line> lines, Pos start, Pos end)
+    {
+        if (lines.Count == 0) return string.Empty;
+        (start, end) = Order(start, end);
+
+        int sl = Math.Clamp(start.Line, 0, lines.Count - 1);
+        int el = Math.Clamp(end.Line, 0, lines.Count - 1);
+
+        if (sl == el)
+        {
+            string t = lines[sl].Text;
+            int c0 = Math.Clamp(start.Col, 0, t.Length);
+            int c1 = Math.Clamp(end.Col, 0, t.Length);
+            if (c1 <= c0) return string.Empty;
+            return t.Substring(c0, c1 - c0);
+        }
+
+        var sb = new StringBuilder();
+
+        // First line: from start.Col to its end.
+        {
+            string t = lines[sl].Text;
+            int c0 = Math.Clamp(start.Col, 0, t.Length);
+            sb.Append(t.AsSpan(c0));
+        }
+
+        // Whole middle lines.
+        for (int i = sl + 1; i < el; i++)
+        {
+            sb.Append('\n');
+            sb.Append(lines[i].Text);
+        }
+
+        // Last line: up to end.Col.
+        {
+            sb.Append('\n');
+            string t = lines[el].Text;
+            int c1 = Math.Clamp(end.Col, 0, t.Length);
+            sb.Append(t.AsSpan(0, c1));
+        }
+
+        return sb.ToString();
+    }
+
+    /// <summary>
+    /// Convert a local-space point to a caret <see cref="Pos"/> against the cached
+    /// layout from the last draw. line = floor((localY - baseY)/lineHeight) clamped
+    /// to the line range; col via <see cref="CharIndexAt"/>.
+    /// </summary>
+    private Pos HitChar(float localX, float localY)
+    {
+        var lines = _lastLines;
+        if (lines.Count == 0) return new Pos(0, 0);
+
+        float lh = _lastLineHeight <= 0f ? 16f : _lastLineHeight;
+        int line = (int)MathF.Floor((localY - _lastBaseY) / lh);
+        line = Math.Clamp(line, 0, lines.Count - 1);
+
+        string text = lines[line].Text;
+        var font = _lastFont;
+        int col = font is null
+            ? 0
+            : CharIndexAt(text, ch => font.TryGetGlyph(ch, out var g) ? g.Advance : 0f,
+                          localX - _lastPadding);
+        return new Pos(line, col);
+    }
+
+    /// <summary>
+    /// The caret column for a horizontal position <paramref name="x"/> (already
+    /// adjusted for the left padding, so x=0 is the start of the text). Walks the
+    /// string accumulating each glyph's advance and snaps the caret to whichever
+    /// side of the glyph midpoint <paramref name="x"/> falls on — natural
+    /// Windows-like caret placement. Pure — unit-testable with a synthetic advance.
+    /// </summary>
+    /// <param name="text">The line text.</param>
+    /// <param name="advanceOf">Per-character advance (pixels) lookup.</param>
+    /// <param name="x">Horizontal position relative to the text's left edge.</param>
+    public static int CharIndexAt(string text, Func<char, float> advanceOf, float x)
+    {
+        if (string.IsNullOrEmpty(text) || x <= 0f) return 0;
+
+        float cursor = 0f;
+        for (int i = 0; i < text.Length; i++)
+        {
+            float adv = advanceOf(text[i]);
+            float mid = cursor + adv * 0.5f;
+            if (x < mid) return i;            // caret sits before this glyph
+            cursor += adv;
+        }
+        return text.Length;                   // past the last glyph → end caret
+    }
 }

src/AcDream.App/UI/UiElement.cs

@@ -102,6 +102,12 @@ public abstract class UiElement
     /// resizes it (window resize). Intended for top-level panels.</summary>
     public bool Resizable { get; set; }
 
+    /// <summary>If true, a left-drag starting on this element is delivered to the
+    /// element (e.g. text selection) instead of moving/resizing an ancestor window.
+    /// Edge resize on a resizable ancestor still wins — only the interior move /
+    /// drag-drop candidacy is suppressed in favour of the element's own handling.</summary>
+    public bool CapturesPointerDrag { get; set; }
+
     /// <summary>Minimum size enforced while resizing.</summary>
     public float MinWidth { get; set; } = 40f;
     public float MinHeight { get; set; } = 40f;

src/AcDream.App/UI/UiHost.cs

@@ -39,6 +39,13 @@ public sealed class UiHost : System.IDisposable
     public UiRoot Root { get; } = new();
     public TextRenderer TextRenderer { get; }
     public BitmapFont? DefaultFont { get; set; }
+
+    /// <summary>The last wired keyboard. Exposed so widgets that need clipboard
+    /// access (<see cref="IKeyboard.ClipboardText"/>) or modifier-key state
+    /// (<see cref="IKeyboard.IsKeyPressed"/>) — e.g. <see cref="UiChatView"/>'s
+    /// Ctrl+C copy — can reach the device. One-keyboard desktop: last wins.</summary>
+    public IKeyboard? Keyboard { get; private set; }
+
     private long _startTicks = System.Environment.TickCount64;
 
     public UiHost(GL gl, string shaderDir, BitmapFont? defaultFont = null)
@@ -82,6 +89,7 @@ public sealed class UiHost : System.IDisposable
 
     public void WireKeyboard(IKeyboard kb)
     {
+        Keyboard = kb;   // last wired keyboard wins (one-keyboard desktop)
         kb.KeyDown += (_, k, _) => Root.OnKeyDown((int)k);
         kb.KeyUp   += (_, k, _) => Root.OnKeyUp((int)k);
         kb.KeyChar += (_, c) => Root.OnChar(c);

src/AcDream.App/UI/UiRoot.cs

@@ -197,7 +197,7 @@ public sealed class UiRoot : UiElement
         if (Modal is not null && !ContainsAbsolute(Modal, x, y))
             return;
 
-        var (target, lx, ly) = HitTestTopDown(x, y);
+        var (target, _, _) = HitTestTopDown(x, y);
         if (target is null)
         {
             WorldMouseFallThrough?.Invoke(btn, x, y, flags);
@@ -218,6 +218,8 @@ public sealed class UiRoot : UiElement
             var edges = window.Resizable ? HitEdges(window, x, y, ResizeGrip) : ResizeEdges.None;
             if (edges != ResizeEdges.None)
             {
+                // Edge resize still wins, even over a CapturesPointerDrag child:
+                // a resizable chat window can be resized from its frame.
                 _resizeTarget = window;
                 _resizeEdges = edges;
                 _resizeStartX = window.Left; _resizeStartY = window.Top;
@@ -225,6 +227,14 @@ public sealed class UiRoot : UiElement
                 _resizeMouseX = x; _resizeMouseY = y;
                 _dragCandidate = false;
             }
+            else if (target.CapturesPointerDrag)
+            {
+                // The pressed widget owns interior drags (e.g. text selection):
+                // do NOT move the ancestor window. The already-dispatched MouseDown
+                // event + SetCapture(target) let the target drive its own drag via
+                // the MouseMove events it receives while captured.
+                _dragCandidate = false;
+            }
             else if (window.Draggable)
             {
                 _windowDragTarget = window;
@@ -234,6 +244,11 @@ public sealed class UiRoot : UiElement
             }
             else { _dragCandidate = true; }
         }
+        else if (target.CapturesPointerDrag)
+        {
+            // No window ancestor, but the target still owns its interior drag.
+            _dragCandidate = false;
+        }
         else
         {
             _dragCandidate = true;
@@ -247,8 +262,13 @@ public sealed class UiRoot : UiElement
             UiMouseButton.Middle => UiEventType.MiddleDown,
             _ => UiEventType.MouseDown,
         };
+        // Deliver TARGET-LOCAL coords (consistent with MouseMove/MouseUp, which use
+        // target.ScreenPosition). HitTestTopDown's lx/ly are relative to the TOP-LEVEL
+        // child, so for a nested target (e.g. the chat view inset inside its window)
+        // they'd be offset by the child's position — which mis-anchored drag-select.
+        var sp = target.ScreenPosition;
         var e = new UiEvent(target.EventId, target, rawType,
-                            Data0: (int)flags, Data1: (int)lx, Data2: (int)ly);
+                            Data0: (int)flags, Data1: (int)(x - sp.X), Data2: (int)(y - sp.Y));
         BubbleEvent(target, in e);
     }
 

tests/AcDream.App.Tests/UI/UiChatViewTests.cs

@@ -1,3 +1,6 @@
+using System;
+using System.Collections.Generic;
+using System.Numerics;
 using AcDream.App.UI;
 
 namespace AcDream.App.Tests.UI;
@@ -25,4 +28,116 @@ public class UiChatViewTests
     {
         Assert.Equal(0f, UiChatView.ClampScroll(-50f, contentHeight: 500f, viewHeight: 200f));
     }
+
+    // ── Char-index hit-testing (x → col) with a synthetic 10px monospace advance ──
+
+    private static readonly Func<char, float> Mono10 = static _ => 10f;
+
+    [Fact]
+    public void CharIndexAt_ZeroOrNegative_IsColumnZero()
+    {
+        Assert.Equal(0, UiChatView.CharIndexAt("hello", Mono10, 0f));
+        Assert.Equal(0, UiChatView.CharIndexAt("hello", Mono10, -5f));
+    }
+
+    [Fact]
+    public void CharIndexAt_SnapsToGlyphMidpoint()
+    {
+        // glyph[0] spans 0..10 (midpoint 5), glyph[1] 10..20 (midpoint 15), ...
+        Assert.Equal(0, UiChatView.CharIndexAt("hello", Mono10, 4f));   // before mid of glyph 0
+        Assert.Equal(1, UiChatView.CharIndexAt("hello", Mono10, 6f));   // past mid of glyph 0
+        Assert.Equal(1, UiChatView.CharIndexAt("hello", Mono10, 14f));  // before mid of glyph 1
+        Assert.Equal(2, UiChatView.CharIndexAt("hello", Mono10, 16f));  // past mid of glyph 1
+    }
+
+    [Fact]
+    public void CharIndexAt_PastEnd_IsLength()
+    {
+        Assert.Equal(5, UiChatView.CharIndexAt("hello", Mono10, 1000f));
+    }
+
+    [Fact]
+    public void CharIndexAt_EmptyString_IsZero()
+    {
+        Assert.Equal(0, UiChatView.CharIndexAt("", Mono10, 50f));
+    }
+
+    // ── SelectedText assembly ────────────────────────────────────────────
+
+    private static IReadOnlyList<UiChatView.Line> Lines(params string[] texts)
+    {
+        var list = new List<UiChatView.Line>(texts.Length);
+        foreach (var t in texts)
+            list.Add(new UiChatView.Line(t, new Vector4(1, 1, 1, 1)));
+        return list;
+    }
+
+    [Fact]
+    public void SelectedText_SingleLine_Substring()
+    {
+        var lines = Lines("hello world");
+        var s = UiChatView.SelectedText(lines, new UiChatView.Pos(0, 6), new UiChatView.Pos(0, 11));
+        Assert.Equal("world", s);
+    }
+
+    [Fact]
+    public void SelectedText_SingleLine_ReversedAnchorCaret_IsNormalised()
+    {
+        var lines = Lines("hello world");
+        // caret BEFORE anchor — Order() must normalise.
+        var s = UiChatView.SelectedText(lines, new UiChatView.Pos(0, 11), new UiChatView.Pos(0, 6));
+        Assert.Equal("world", s);
+    }
+
+    [Fact]
+    public void SelectedText_SamePosition_IsEmpty()
+    {
+        var lines = Lines("hello");
+        Assert.Equal("", UiChatView.SelectedText(lines, new UiChatView.Pos(0, 3), new UiChatView.Pos(0, 3)));
+    }
+
+    [Fact]
+    public void SelectedText_MultiLine_JoinsWithNewline()
+    {
+        var lines = Lines("first line", "second line", "third line");
+        // from col 6 of line 0 ("line") through col 5 of line 2 ("third")
+        var s = UiChatView.SelectedText(lines, new UiChatView.Pos(0, 6), new UiChatView.Pos(2, 5));
+        Assert.Equal("line\nsecond line\nthird", s);
+    }
+
+    [Fact]
+    public void SelectedText_MultiLine_TwoLines_NoMiddle()
+    {
+        var lines = Lines("alpha", "bravo");
+        var s = UiChatView.SelectedText(lines, new UiChatView.Pos(0, 2), new UiChatView.Pos(1, 3));
+        Assert.Equal("pha\nbra", s);
+    }
+
+    [Fact]
+    public void SelectedText_MultiLine_ReversedAnchorCaret_IsNormalised()
+    {
+        var lines = Lines("alpha", "bravo");
+        // end before start → Order() swaps them.
+        var s = UiChatView.SelectedText(lines, new UiChatView.Pos(1, 3), new UiChatView.Pos(0, 2));
+        Assert.Equal("pha\nbra", s);
+    }
+
+    [Fact]
+    public void SelectedText_EmptyLineList_IsEmpty()
+    {
+        Assert.Equal("", UiChatView.SelectedText(Array.Empty<UiChatView.Line>(),
+            new UiChatView.Pos(0, 0), new UiChatView.Pos(0, 0)));
+    }
+
+    [Fact]
+    public void Order_SortsByLineThenColumn()
+    {
+        var (s1, e1) = UiChatView.Order(new UiChatView.Pos(2, 1), new UiChatView.Pos(0, 5));
+        Assert.Equal(new UiChatView.Pos(0, 5), s1);
+        Assert.Equal(new UiChatView.Pos(2, 1), e1);
+
+        var (s2, e2) = UiChatView.Order(new UiChatView.Pos(1, 8), new UiChatView.Pos(1, 2));
+        Assert.Equal(new UiChatView.Pos(1, 2), s2);
+        Assert.Equal(new UiChatView.Pos(1, 8), e2);
+    }
 }

tests/AcDream.App.Tests/UI/UiRootInputTests.cs

@@ -14,6 +14,41 @@ public class UiRootInputTests
         Assert.Equal(AnchorEdges.None, panel.Anchors);
     }
 
+    private sealed class CoordRecorder : UiElement
+    {
+        public (int x, int y)? Down, Move;
+        public CoordRecorder() { CapturesPointerDrag = true; }
+        public override bool OnEvent(in UiEvent e)
+        {
+            if (e.Type == UiEventType.MouseDown) { Down = (e.Data1, e.Data2); return true; }
+            if (e.Type == UiEventType.MouseMove) { Move = (e.Data1, e.Data2); return true; }
+            return false;
+        }
+    }
+
+    [Fact]
+    public void MouseDown_And_MouseMove_DeliverSameTargetLocalFrame_ForNestedChild()
+    {
+        // Regression (adversarial review): a nested child must receive target-LOCAL
+        // coords on MouseDown AND MouseMove for the same physical point — otherwise
+        // drag-select anchors ~(child offset) px off from where you click. Before the
+        // fix MouseDown used HitTestTopDown's window-relative coords (50,40) while
+        // MouseMove used target-local (42,32).
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var panel = new UiPanel { Left = 50, Top = 60, Width = 200, Height = 100 };
+        var child = new CoordRecorder { Left = 8, Top = 8, Width = 150, Height = 80 };
+        panel.AddChild(child);
+        root.AddChild(panel);
+
+        // child ScreenPosition = (58,68). Click screen (100,100) -> local (42,32).
+        root.OnMouseDown(UiMouseButton.Left, 100, 100);
+        Assert.Equal((42, 32), child.Down);
+
+        // drag to (120,110) -> local (62,42); MUST share the MouseDown frame.
+        root.OnMouseMove(120, 110);
+        Assert.Equal((62, 42), child.Move);
+    }
+
     [Fact]
     public void ApplyAnchor_None_IsNoOp()
     {
@@ -70,6 +105,54 @@ public class UiRootInputTests
         Assert.Equal(10f, panel.Top);
     }
 
+    [Fact]
+    public void CapturesPointerDragChild_DoesNotMoveDraggableAncestor_OnInteriorDrag()
+    {
+        // A child that captures pointer drags (text selection) must NOT move its
+        // draggable ancestor window when the user drags inside it.
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var window = new UiPanel { Left = 10, Top = 10, Width = 200, Height = 100, Draggable = true };
+        var child = new UiPanel { Left = 20, Top = 20, Width = 120, Height = 60, CapturesPointerDrag = true };
+        window.AddChild(child);
+        root.AddChild(window);
+
+        // Press deep inside the child, then drag.
+        root.OnMouseDown(UiMouseButton.Left, 60, 60);
+        root.OnMouseMove(160, 160);
+
+        // Window stays put; the captured child receives the drag itself.
+        Assert.Equal(10f, window.Left);
+        Assert.Equal(10f, window.Top);
+        Assert.Same(child, root.Captured);
+
+        root.OnMouseUp(UiMouseButton.Left, 160, 160);
+        Assert.Equal(10f, window.Left);
+        Assert.Equal(10f, window.Top);
+    }
+
+    [Fact]
+    public void CapturesPointerDragChild_StillAllowsEdgeResizeOfResizableWindow()
+    {
+        // Edge resize must still win even when a CapturesPointerDrag child covers
+        // the frame: a resizable chat window can be resized from its border.
+        var root = new UiRoot { Width = 800, Height = 600 };
+        var window = new UiPanel { Left = 100, Top = 100, Width = 200, Height = 100,
+            Draggable = true, Resizable = true, MinWidth = 40, MinHeight = 40 };
+        // Child fills the whole window (anchored) and captures interior drags.
+        var child = new UiPanel { Left = 0, Top = 0, Width = 200, Height = 100,
+            CapturesPointerDrag = true,
+            Anchors = AnchorEdges.Left | AnchorEdges.Top | AnchorEdges.Right | AnchorEdges.Bottom };
+        window.AddChild(child);
+        root.AddChild(window);
+
+        // Grab within ResizeGrip(5) of the right edge (x=298 of right edge x=300) → resize.
+        root.OnMouseDown(UiMouseButton.Left, 298, 150);
+        root.OnMouseMove(338, 150);
+        Assert.Equal(240f, window.Width);
+        Assert.Equal(100f, window.Left);
+        root.OnMouseUp(UiMouseButton.Left, 338, 150);
+    }
+
     [Fact]
     public void ResizeRect_RightBottom_GrowsSizeOnly()
     {