Popups: reworked GetMousePosOnOpeningCurrentPopup() to be legal to call within the BeginPopup/EndPopup scope of a popup which was just closed. (#9551)

docs/CHANGELOG.txt

@@ -83,6 +83,8 @@ Other Changes:
   - Amended context menu right-click opening code to check that the right-click
     hasn't been owned by another items. This is more correct and also necessary
     to avoid Drags/Sliders cancelling also triggering a cancel menu. (#8564, #9534)
+  - Reworked `GetMousePosOnOpeningCurrentPopup()` to be legal to call within
+    the BeginPopup/EndPopup scope of a popup which was just closed. (#9551) [@jkunstwald]
 - ImGuiTextFilter:
   - Added support for space as an "and" filter, which is the standard. (#2435, #30)
      e.g. w/ filter 'hello world' (without quotes) both words needs to be in input data.

imgui.cpp

@@ -10269,12 +10269,11 @@ void ImGui::TeleportMousePos(const ImVec2& pos)
     //IMGUI_DEBUG_LOG_IO("TeleportMousePos: (%.1f,%.1f)\n", io.MousePos.x, io.MousePos.y);
 }
 
-// NB: prefer to call right after BeginPopup(). At the time Selectable/MenuItem is activated, the popup is already closed!
 ImVec2 ImGui::GetMousePosOnOpeningCurrentPopup()
 {
     ImGuiContext& g = *GImGui;
     if (g.BeginPopupStack.Size > 0)
-        return g.OpenPopupStack[g.BeginPopupStack.Size - 1].OpenMousePos;
+        return g.BeginPopupStack[g.BeginPopupStack.Size - 1].OpenMousePos;
     return g.IO.MousePos;
 }