feat(ui): add MessageList with two-mode virtualisation

Compact mode reuses ImGuiListClipper because rows are a constant line
height; card mode falls back to a linear render with a per-message height
cache and an IsItemVisible skip path so off-screen rows place a Dummy of
the cached height instead of running the full render. Bottom-lock detects
whether the user was pinned to the bottom before the layout pass and
re-pins after new rows land. Renders text-only via SeString.TextValue for
this cycle — full chunk and payload rendering re-attaches later, so the
component shape stays correct without dragging the v1.5.6 chunk pipeline
into the new layer.

HellionChat/PluginHostFactory.cs

@@ -129,6 +129,11 @@ internal static class PluginHostFactory
             sp.GetRequiredService<FontManager>(),
             sp.GetRequiredService<ILogger<Ui.Components.Sidebar>>()
         ));
+        services.AddSingleton(sp => new Ui.Components.MessageList(
+            sp.GetRequiredService<ThemeRegistry>(),
+            sp.GetRequiredService<Ui.StyleEngine.TokenResolver>(),
+            sp.GetRequiredService<FontManager>()
+        ));
         services.AddSingleton(sp => new Integrations.FailedTellNotifier(
             sp.GetRequiredService<ILogger<Integrations.FailedTellNotifier>>()
         ));

HellionChat/Ui/Components/MessageList.cs

@@ -0,0 +1,147 @@
+using System.Globalization;
+using System.Numerics;
+using Dalamud.Bindings.ImGui;
+using Dalamud.Interface.Utility.Raii;
+using HellionChat.Themes;
+using HellionChat.Ui.StyleEngine;
+using HellionChat.Util;
+
+namespace HellionChat.Ui.Components;
+
+// Virtualised message list. Compact mode reuses ImGuiListClipper because
+// rows have a constant line height; card mode falls back to a linear
+// render with a per-message height cache and an IsItemVisible skip path
+// so off-screen rows place a Dummy of the cached height rather than
+// running the full render again. Text-only rendering for now — full
+// chunk/payload rendering re-attaches in a later cycle.
+internal sealed class MessageList
+{
+    private const float CompactRowHeight = 18f;
+
+    private readonly ThemeRegistry _themes;
+    private readonly TokenResolver _resolver;
+    private readonly FontManager _fonts;
+
+    public MessageList(ThemeRegistry themes, TokenResolver resolver, FontManager fonts)
+    {
+        _themes = themes;
+        _resolver = resolver;
+        _fonts = fonts;
+    }
+
+    public void Draw(Tab tab)
+    {
+        if (!_fonts.FontsReady)
+        {
+            ImGui.TextUnformatted("Loading fonts…");
+            return;
+        }
+
+        using var child = ImRaii.Child("##hellion-messages", new Vector2(-1, -1));
+        if (!child.Success)
+            return;
+
+        var theme = _themes.Active;
+        var textAbgr = ColourUtil.RgbaToAbgr(theme.Colors.TextPrimary);
+        var mutedAbgr = ColourUtil.RgbaToAbgr(theme.Colors.TextMuted);
+
+        using var messages = tab.Messages.GetReadOnly(3);
+        var compact = Plugin.Config.UseCompactDensity;
+
+        // Track whether the user was pinned to the bottom before this frame
+        // so newly arriving rows do not yank them up — the standard
+        // chat-window expectation. Read the scroll state before drawing
+        // anything inside the child so the comparison is against the
+        // previous frame's max.
+        var pinnedToBottom = ImGui.GetScrollY() >= ImGui.GetScrollMaxY() - 1f;
+
+        if (compact)
+            DrawCompact(messages, textAbgr, mutedAbgr);
+        else
+            DrawCard(tab, messages, textAbgr, mutedAbgr);
+
+        if (pinnedToBottom)
+            ImGui.SetScrollHereY(1f);
+    }
+
+    private void DrawCompact(IReadOnlyList<Message> messages, uint textAbgr, uint mutedAbgr)
+    {
+        unsafe
+        {
+            var clipper = new ImGuiListClipperPtr(ImGuiNative.ImGuiListClipper());
+            try
+            {
+                clipper.Begin(messages.Count, CompactRowHeight);
+                while (clipper.Step())
+                {
+                    for (var i = clipper.DisplayStart; i < clipper.DisplayEnd; i++)
+                        DrawCompactRow(messages[i], textAbgr, mutedAbgr);
+                }
+                clipper.End();
+            }
+            finally
+            {
+                clipper.Destroy();
+            }
+        }
+    }
+
+    private void DrawCompactRow(Message message, uint textAbgr, uint mutedAbgr)
+    {
+        var timestamp = FormatTimestamp(message.Date);
+        var sender = message.SenderSource.TextValue;
+        var content = message.ContentSource.TextValue;
+        var line = string.IsNullOrEmpty(sender)
+            ? $"{timestamp}  {content}"
+            : $"{timestamp}  {sender}: {content}";
+        ImGui.TextUnformatted(line);
+    }
+
+    private void DrawCard(Tab tab, IReadOnlyList<Message> messages, uint textAbgr, uint mutedAbgr)
+    {
+        var tabId = tab.Identifier;
+        for (var i = 0; i < messages.Count; i++)
+        {
+            var msg = messages[i];
+
+            // Cached row: place a Dummy of the known height and skip the
+            // full render path if the row is off-screen. Mirrors the
+            // v1.5.6 Card-Mode pattern in ChatLogWindow.DrawMessages.
+            msg.Height.TryGetValue(tabId, out var cachedHeight);
+            if (cachedHeight is float h)
+            {
+                var beforeDummy = ImGui.GetCursorPos();
+                ImGui.Dummy(new Vector2(10f, h));
+                var visible = ImGui.IsItemVisible();
+                msg.IsVisible[tabId] = visible;
+                if (!visible)
+                    continue;
+                ImGui.SetCursorPos(beforeDummy);
+            }
+
+            var before = ImGui.GetCursorPosY();
+            DrawCardRow(msg);
+            var after = ImGui.GetCursorPosY();
+            msg.Height[tabId] = after - before;
+        }
+    }
+
+    private void DrawCardRow(Message message)
+    {
+        var timestamp = FormatTimestamp(message.Date);
+        var sender = message.SenderSource.TextValue;
+        var content = message.ContentSource.TextValue;
+        ImGui.TextUnformatted(string.IsNullOrEmpty(sender) ? timestamp : $"{timestamp}  {sender}");
+        ImGui.PushTextWrapPos(0f);
+        ImGui.TextUnformatted(content);
+        ImGui.PopTextWrapPos();
+    }
+
+    private static string FormatTimestamp(DateTimeOffset date)
+    {
+        var local = date.ToLocalTime();
+        return Plugin.Config.Use24HourClock
+            ? local.ToString("HH:mm", CultureInfo.InvariantCulture)
+            : local.ToString("h:mm tt", CultureInfo.InvariantCulture);
+    }
+}