Viewports: Improve menu positioning in multi-monitor setups.

This change uses mouse position for initial positioning of popup menus. It ensures that menu appears in expected location on monitor mouse is currently hovering. This change fixes incorrect menu positioning (permanent or on the first frame) when menu item spans multiple monitors.

imgui.cpp

@@ -10986,7 +10986,7 @@ static void ImGui::UpdateSelectWindowViewport(ImGuiWindow* window)
             ImVec2 mouse_ref = (flags & ImGuiWindowFlags_Tooltip) ? g.IO.MousePos : g.BeginPopupStack.back().OpenMousePos;
             bool use_mouse_ref = (g.NavDisableHighlight || !g.NavDisableMouseHover || !g.NavWindow);
             bool mouse_valid = IsMousePosValid(&mouse_ref);
-            if ((window->Appearing || (flags & ImGuiWindowFlags_Tooltip)) && (!use_mouse_ref || mouse_valid))
+            if ((window->Appearing || (flags & (ImGuiWindowFlags_Tooltip | ImGuiWindowFlags_ChildMenu))) && (!use_mouse_ref || mouse_valid))
                 window->ViewportAllowPlatformMonitorExtend = FindPlatformMonitorForPos((use_mouse_ref && mouse_valid) ? mouse_ref : NavCalcPreferredRefPos());
             else
                 window->ViewportAllowPlatformMonitorExtend = window->Viewport->PlatformMonitor;