Popups: Fixed an issue when reopening a same popup multiple times would offset them by 1 pixel on the right. (#4936)

Passing explicit ImGuiPopupFlags_MouseButtonRight to OpenPopupOnItemClick() calls somehow document the unusual (due to legacy) default value.

imgui.h

@@ -681,6 +681,7 @@ namespace ImGui
     //  - CloseCurrentPopup() is called by default by Selectable()/MenuItem() when activated (FIXME: need some options).
     //  - Use ImGuiPopupFlags_NoOpenOverExistingPopup to avoid opening a popup if there's already one at the same level. This is equivalent to e.g. testing for !IsAnyPopupOpen() prior to OpenPopup().
     //  - Use IsWindowAppearing() after BeginPopup() to tell if a window just opened.
+    //  - IMPORTANT: Notice that for OpenPopupOnItemClick() we exceptionally default flags to 1 (== ImGuiPopupFlags_MouseButtonRight) for backward compatibility with older API taking 'int mouse_button = 1' parameter
     IMGUI_API void          OpenPopup(const char* str_id, ImGuiPopupFlags popup_flags = 0);                     // call to mark popup as open (don't call every frame!).
     IMGUI_API void          OpenPopup(ImGuiID id, ImGuiPopupFlags popup_flags = 0);                             // id overload to facilitate calling from nested stacks
     IMGUI_API void          OpenPopupOnItemClick(const char* str_id = NULL, ImGuiPopupFlags popup_flags = 1);   // helper to open popup when clicked on last item. Default to ImGuiPopupFlags_MouseButtonRight == 1. (note: actually triggers on the mouse _released_ event to be consistent with popup behaviors)
@@ -690,7 +691,7 @@ namespace ImGui
     //  - Helpers to do OpenPopup+BeginPopup where the Open action is triggered by e.g. hovering an item and right-clicking.
     //  - They are convenient to easily create context menus, hence the name.
     //  - IMPORTANT: Notice that BeginPopupContextXXX takes ImGuiPopupFlags just like OpenPopup() and unlike BeginPopup(). For full consistency, we may add ImGuiWindowFlags to the BeginPopupContextXXX functions in the future.
-    //  - IMPORTANT: we exceptionally default their flags to 1 (== ImGuiPopupFlags_MouseButtonRight) for backward compatibility with older API taking 'int mouse_button = 1' parameter, so if you add other flags remember to re-add the ImGuiPopupFlags_MouseButtonRight.
+    //  - IMPORTANT: Notice that we exceptionally default their flags to 1 (== ImGuiPopupFlags_MouseButtonRight) for backward compatibility with older API taking 'int mouse_button = 1' parameter, so if you add other flags remember to re-add the ImGuiPopupFlags_MouseButtonRight.
     IMGUI_API bool          BeginPopupContextItem(const char* str_id = NULL, ImGuiPopupFlags popup_flags = 1);  // open+begin popup when clicked on last item. Use str_id==NULL to associate the popup to previous item. If you want to use that on a non-interactive item such as Text() you need to pass in an explicit ID here. read comments in .cpp!
     IMGUI_API bool          BeginPopupContextWindow(const char* str_id = NULL, ImGuiPopupFlags popup_flags = 1);// open+begin popup when clicked on current window.
     IMGUI_API bool          BeginPopupContextVoid(const char* str_id = NULL, ImGuiPopupFlags popup_flags = 1);  // open+begin popup when clicked in void (where there are no windows).