OpenPopupOnItemClick: add optional `bool* p_open`

[Glordim]

Version/Branch of Dear ImGui:

Version 1.9X, Branch: master

Details:

Context

I have a context menu popup that must open both when clicking on a list item and when clicking on the empty area of the container. Because of this dual trigger, I cannot use BeginPopupContextItem and BeginPopupContextWindow together on the same popup ID.

Problem

For the item-click case I would naturally use OpenPopupOnItemClick, but I also need to capture which item was clicked so I can prepare the context (e.g. _contextItem = item) at the moment the popup opens — not every frame inside BeginPopup.

Before 1.79 I could write:

if (ImGui::OpenPopupOnItemClick("##ctx"))
    _contextItem = item;

This was removed in 1.79 with a good rationale about API consistency.

Proposal

1. Add an optional bool* p_open out-param to OpenPopupOnItemClick.
2. Introduce OpenPopupOnWindowClick.
3. Introduce OpenPopupOnVoidClick.

IMGUI_API void OpenPopupOnItemClick(const char* str_id = NULL,
                                    ImGuiPopupFlags popup_flags = 1,
                                    bool* p_open= NULL);

IMGUI_API void OpenPopupOnWindowClick(const char* str_id = NULL,
                                      ImGuiPopupFlags popup_flags = 1,
                                      bool* p_open= NULL);

IMGUI_API void OpenPopupOnVoidClick(const char* str_id = NULL,
                                    ImGuiPopupFlags popup_flags = 1,
                                    bool* p_open= NULL);

p_open would be set to true only on the frame the popup transitions from closed to open, and false otherwise — a one-shot event, consistent with the p_open pattern used elsewhere in the API.

Usage:

for (const auto& item : items)
{
    ImGui::Selectable(item.name);

    bool open = false;
    ImGui::OpenPopupOnItemClick("##ctx", ImGuiPopupFlags_MouseButtonRight, &open);
    if (open)
        _contextItem = &item;
}

bool open = false;
ImGui::OpenPopupOnWindowClick("##ctx", ImGuiPopupFlags_MouseButtonRight, &open);
if (open)
    _contextItem = nullptr;

if (ImGui::BeginPopup("##ctx"))
{
    // ...
    ImGui::EndPopup();
}

This keeps full backward compatibility and allows mixing both triggers on the same popup ID without resorting to manual IsItemHovered + OpenPopup calls.

Happy to submit a PR for this if the proposal looks good to you.