Menus: Tweak for first-level menu windows to be positioned regardless of MenuBarHeight() which is not stored in window, allowing undocumented modification of the menu bar height).

docs/CHANGELOG.txt

@@ -40,7 +40,7 @@ Other Changes:
   branch pressing arrow keys while dragging a window from a tab could trigger an assert. (#3025)
 - ColorButton: Added ImGuiColorEditFlags_NoBorder flag to remove the border normally enforced
   by default for standalone ColorButton.
-- BeginMenu: Using same ID multiple times appends content to a menu. (#1207)
+- BeginMenu: Using same ID multiple times appends content to a menu. (#1207) [@rokups]
 - BeginMenu: Fixed a bug where SetNextWindowXXX data before a BeginMenu() would not be cleared
   when the menu is not open. (#3030)
 - InputText: Fixed password fields displaying ASCII spaces as blanks instead of using the '*'

imgui.cpp

@@ -7830,7 +7830,7 @@ ImVec2 ImGui::FindBestWindowPosForPopup(ImGuiWindow* window)
         float horizontal_overlap = g.Style.ItemInnerSpacing.x; // We want some overlap to convey the relative depth of each menu (currently the amount of overlap is hard-coded to style.ItemSpacing.x).
         ImRect r_avoid;
         if (parent_window->DC.MenuBarAppending)
-            r_avoid = ImRect(-FLT_MAX, parent_window->Pos.y + parent_window->TitleBarHeight(), FLT_MAX, parent_window->Pos.y + parent_window->TitleBarHeight() + parent_window->MenuBarHeight());
+            r_avoid = ImRect(-FLT_MAX, parent_window->ClipRect.Min.y, FLT_MAX, parent_window->ClipRect.Max.y); // Avoid parent menu-bar. If we wanted multi-line menu-bar, we may instead want to have the calling window setup e.g. a NextWindowData.PosConstraintAvoidRect field
         else
             r_avoid = ImRect(parent_window->Pos.x + horizontal_overlap, -FLT_MAX, parent_window->Pos.x + parent_window->Size.x - horizontal_overlap - parent_window->ScrollbarSizes.x, FLT_MAX);
         return FindBestWindowPosForPopupEx(window->Pos, window->Size, &window->AutoPosLastDirection, r_outer, r_avoid);