Viewport: Popups by default merge into parent/host viewport as they have no decoration (same as menu/child). (#1542)

imgui.cpp

@@ -7754,7 +7754,7 @@ static bool ImGui::GetWindowAlwaysWantOwnViewport(ImGuiWindow* window)
     ImGuiContext& g = *GImGui;
     if (g.IO.ConfigViewportsNoAutoMerge && (g.ConfigFlagsForFrame & ImGuiConfigFlags_ViewportsEnable))
         if (!window->DockIsActive)
-            if ((window->Flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_ChildMenu | ImGuiWindowFlags_Tooltip)) == 0)
+            if ((window->Flags & (ImGuiWindowFlags_ChildWindow | ImGuiWindowFlags_ChildMenu | ImGuiWindowFlags_Tooltip | ImGuiWindowFlags_Popup)) == 0)
                 return true;
     return false;
 }