feat(messages): restore scroll-to-bottom bar with snap decision

HellionChat/Ui/Components/MessageList.cs

@@ -1,7 +1,8 @@
 using System.Globalization;
 using System.Numerics;
 using Dalamud.Bindings.ImGui;
-using Dalamud.Interface.Utility.Raii;
+using Dalamud.Interface.Utility;
+using HellionChat.Resources;
 using HellionChat.Util;
 
 namespace HellionChat.Ui.Components;
@@ -20,6 +21,12 @@ internal sealed class MessageList
 
     private PayloadHandler? _handler;
 
+    // B3-5: scroll-to-bottom state. Per-instance, so pop-out windows (own
+    // MessageList instance, PluginHostFactory.cs:263-266) isolate automatically —
+    // the old 1.5.6 updateScrollState flag is NOT needed here.
+    private bool _scrolledUp;
+    private bool _scrollToBottomRequested;
+
     // §6.2: setter-injection breaks the PayloadHandler → MainWindow → MessageList → PayloadHandler 3-cycle.
     // Wired by PayloadHandlerInitHostedService.StartAsync after both singletons exist.
     internal void AttachPayloadHandler(PayloadHandler handler)
@@ -33,6 +40,20 @@ internal sealed class MessageList
         _chunkRenderer = chunkRenderer;
     }
 
+    // Deterministic and ImGui-free: encapsulates the snap decision AND the
+    // request reset, so the reset invariant is covered. Called by the real Draw.
+    internal bool ResolveSnapToBottom(bool pinnedToBottom)
+    {
+        var snap = pinnedToBottom || _scrollToBottomRequested;
+        _scrollToBottomRequested = false;
+        return snap;
+    }
+
+    // SelfTest hook (B3-5 reset-invariant, REQUIRED — not optional). Lets
+    // ScrollSnapDecisionStep flip the request flag without a real click, so the
+    // post-snap reset can be asserted; without it only the OR branch is testable.
+    internal void RequestScrollToBottomForSelfTest() => _scrollToBottomRequested = true;
+
     public void Draw(Tab tab)
     {
         if (!_fonts.FontsReady)
@@ -57,13 +78,71 @@ internal sealed class MessageList
         else
             DrawCard(tab, messages);
 
-        if (pinnedToBottom)
+        // B3-5: scroll values are frame-constant inside the child, so this
+        // reflects the current frame's state wherever it runs; kept after the
+        // render to mirror the 1.5.6 end-of-DrawMessageLog placement.
+        _scrolledUp = ImGui.GetScrollMaxY() - ImGui.GetScrollY() > 1f;
+
+        if (ResolveSnapToBottom(pinnedToBottom))
             ImGui.SetScrollHereY(1f);
 
+        DrawScrollToBottomBar();
+
         // OpenPopup in Click() and BeginPopup here share the ##hellion-main-area scope -> Popup-ID matches.
         _handler?.Draw();
     }
 
+    // B3-5: Discord-style full-width bar pinned to the bottom edge of the
+    // visible region while the user is scrolled up. Geometry comes from window
+    // pos + size (visible region), never from the content flow: when scrolled
+    // up the visible bottom sits above the content bottom, so the
+    // InvisibleButton stays inside the existing content rect and cannot grow
+    // GetScrollMaxY(). Drawn on the WINDOW drawlist so the enclosing child
+    // clips it; submitted after every payload chunk so the button wins the
+    // hit-test and PostPayload clicks underneath do not double-fire.
+    private void DrawScrollToBottomBar()
+    {
+        if (!_scrolledUp)
+            return;
+
+        var winPos = ImGui.GetWindowPos();
+        var winSize = ImGui.GetWindowSize();
+        var barHeight = ImGui.GetFrameHeight();
+        // The bar only renders while content overflows, so the vertical
+        // scrollbar is always up — keep the bar clear of it.
+        var barWidth = winSize.X - ImGui.GetStyle().ScrollbarSize;
+        var barTopLeft = new Vector2(winPos.X, winPos.Y + winSize.Y - barHeight);
+        var barBottomRight = barTopLeft + new Vector2(barWidth, barHeight);
+
+        var theme = Plugin.Instance.ThemeRegistry.Active;
+        var hovered = ImGui.IsMouseHoveringRect(barTopLeft, barBottomRight);
+        var fill = ColourUtil.RgbaToAbgr(
+            hovered ? theme.Colors.SurfaceHover : theme.Colors.Surface
+        );
+        var rounding = 4f * ImGuiHelpers.GlobalScale;
+        var dl = ImGui.GetWindowDrawList();
+        dl.AddRectFilled(barTopLeft, barBottomRight, fill, rounding);
+        dl.AddRect(
+            barTopLeft,
+            barBottomRight,
+            ColourUtil.RgbaToAbgr(theme.Colors.Border),
+            rounding
+        );
+
+        var label = HellionStrings.ChatLog_ScrollToBottom_Tooltip;
+        var textSize = ImGui.CalcTextSize(label);
+        var textPos =
+            barTopLeft + new Vector2((barWidth - textSize.X) / 2f, (barHeight - textSize.Y) / 2f);
+        dl.AddText(textPos, ColourUtil.RgbaToAbgr(theme.Colors.Accent), label);
+
+        // Click target after the visuals; nothing advances the cursor past the
+        // button, so content height is identical with and without the bar.
+        ImGui.SetCursorScreenPos(barTopLeft);
+        ImGui.InvisibleButton("##scroll-to-bottom-bar", new Vector2(barWidth, barHeight));
+        if (ImGui.IsItemClicked())
+            _scrollToBottomRequested = true;
+    }
+
     private void DrawCompact(IReadOnlyList<Message> messages)
     {
         unsafe

HellionChat/Ui/Windows/MainWindow.cs

@@ -125,6 +125,8 @@ internal sealed class MainWindow : Window
 
     internal Components.HonorificHeader GetHonorificHeaderForSelfTest() => _honorific;
 
+    internal Components.MessageList GetMessageListForSelfTest() => _messages;
+
     // new-shadow on Window.Toggle so the open path also writes Config —
     // OnClose already covers the close path through the base behaviour.
     public new void Toggle()