ImDrawList: Automatically calling ChannelsMerge() if not done after a split.

Ties fixing f422e78 in docking: iterating root windows was incorrect. Will be faster to lazily merge in RenderDimmedBackground() rather than iterate all.

docs/CHANGELOG.txt

@@ -63,6 +63,7 @@ Other changes:
 - Clipper: Added IncludeItemByIndex() helper to include a single item. (#6424, #3841)
 - ImDrawData: Fixed an issue where TotalVtxCount/TotalIdxCount does not match the sum
   of individual ImDrawList's buffer sizes when a dimming/modal background is rendered. (#6716)
+- ImDrawList: Automatically calling ChannelsMerge() if not done after a split.
 - ImDrawList: Fixed OOB access in _CalcCircleAutoSegmentCount when passing excessively
   large radius to AddCircle(). (#6657, #5317) [@EggsyCRO, @jdpatdiscord]
 - IO: Exposed io.PlatformLocaleDecimalPoint to configure decimal point ('.' or ',') for

imgui.cpp

@@ -4812,6 +4812,8 @@ static void AddWindowToDrawData(ImGuiWindow* window, int layer)
     ImGuiContext& g = *GImGui;
     ImGuiViewportP* viewport = g.Viewports[0];
     g.IO.MetricsRenderWindows++;
+    if (window->DrawList->_Splitter._Count > 1)
+        window->DrawList->ChannelsMerge(); // Merge if user forgot to merge back. Also required in Docking branch for ImGuiWindowFlags_DockNodeHost windows.
     ImGui::AddDrawListToDrawDataEx(&viewport->DrawDataP, viewport->DrawDataBuilder.Layers[layer], window->DrawList);
     for (ImGuiWindow* child : window->DC.ChildWindows)
         if (IsWindowActiveAndVisible(child)) // Clipped children may have been marked not active