Viewport: Popups and Tooltips viewports are correctly parented to the parent window's viewport. (#2409, #1542)

imgui.cpp

@@ -5623,6 +5623,8 @@ bool ImGui::Begin(const char* name, bool* p_open, ImGuiWindowFlags flags)
             // We don't default to the main viewport because.
             if (window->WindowClass.ParentViewportId)
                 window->Viewport->ParentViewportId = window->WindowClass.ParentViewportId;
+            else if ((flags & (ImGuiWindowFlags_Popup | ImGuiWindowFlags_Tooltip)) && parent_window_in_stack)
+                window->Viewport->ParentViewportId = parent_window_in_stack->Viewport->ID;
             else
                 window->Viewport->ParentViewportId = g.IO.ConfigViewportsNoDefaultParent ? 0 : IMGUI_VIEWPORT_DEFAULT_ID;
             if (window->WindowClass.ViewportFlagsOverrideMask)