Menus: Fix appending to main menubar (ocornut#3068).

Author: rokups

imgui_widgets.cpp

@@ -6150,27 +6150,34 @@ bool ImGui::BeginMainMenuBar()
 {
     ImGuiContext& g = *GImGui;
     ImGuiViewportP* viewport = g.Viewports[0];
+    ImGuiWindow* menu_bar_window = FindWindowByName("##MainMenuBar");
 
     // For the main menu bar, which cannot be moved, we honor g.Style.DisplaySafeAreaPadding to ensure text can be visible on a TV set.
     g.NextWindowData.MenuBarOffsetMinVal = ImVec2(g.Style.DisplaySafeAreaPadding.x, ImMax(g.Style.DisplaySafeAreaPadding.y - g.Style.FramePadding.y, 0.0f));
 
-    // Get our rectangle in the work area, and report the size we need for next frame.
-    // We don't attempt to calculate our height ahead, as it depends on the per-viewport font size. However menu-bar will affect the minimum window size so we'll get the right height.
-    ImVec2 menu_bar_pos = viewport->Pos + viewport->CurrWorkOffsetMin;
-    ImVec2 menu_bar_size = ImVec2(viewport->Size.x - viewport->CurrWorkOffsetMin.x + viewport->CurrWorkOffsetMax.x, 1.0f);
+    // Get our rectangle at the top of the work area
+    if (menu_bar_window == NULL || menu_bar_window->BeginCount == 0)
+    {
+        // Set window position
+        // We don't attempt to calculate our height ahead, as it depends on the per-viewport font size. However menu-bar will affect the minimum window size so we'll get the right height.
+        ImVec2 menu_bar_pos = viewport->Pos + viewport->CurrWorkOffsetMin;
+        ImVec2 menu_bar_size = ImVec2(viewport->Size.x - viewport->CurrWorkOffsetMin.x + viewport->CurrWorkOffsetMax.x, 1.0f);
+        SetNextWindowPos(menu_bar_pos);
+        SetNextWindowSize(menu_bar_size);
+    }
 
     // Create window
-    SetNextWindowPos(menu_bar_pos);
-    SetNextWindowSize(menu_bar_size);
     SetNextWindowViewport(viewport->ID); // Enforce viewport so we don't create our own viewport when ImGuiConfigFlags_ViewportsNoMerge is set.
     PushStyleVar(ImGuiStyleVar_WindowRounding, 0.0f);
     PushStyleVar(ImGuiStyleVar_WindowMinSize, ImVec2(0, 0));    // Lift normal size constraint, however the presence of a menu-bar will give us the minimum height we want.
     ImGuiWindowFlags window_flags = ImGuiWindowFlags_NoDocking | ImGuiWindowFlags_NoTitleBar | ImGuiWindowFlags_NoResize | ImGuiWindowFlags_NoMove | ImGuiWindowFlags_NoScrollbar | ImGuiWindowFlags_NoSavedSettings | ImGuiWindowFlags_MenuBar;
     bool is_open = Begin("##MainMenuBar", NULL, window_flags) && BeginMenuBar();
     PopStyleVar(2);
 
-    // Feed back into work area using actual window size
-    viewport->CurrWorkOffsetMin.y += GetCurrentWindow()->Size.y;
+    // Report our size into work area (for next frame) using actual window size
+    menu_bar_window = GetCurrentWindow();
+    if (menu_bar_window->BeginCount == 1)
+        viewport->CurrWorkOffsetMin.y += menu_bar_window->Size.y;
 
     g.NextWindowData.MenuBarOffsetMinVal = ImVec2(0.0f, 0.0f);
     if (!is_open)