Begin() to append doesn't set LastItemData for title-bar/tab (was BeginItemTooltip may react to wrong widgets?)

[achabense]

Version/Branch of Dear ImGui:

Version 1.90.5

Back-ends:

imgui_impl_sdl2.cpp + imgui_impl_sdlrenderer2.cpp

Compiler, OS:

Windows 10 + MSVC 2022

Full config/build information:

No response

Details:

When the window is collapsed, BeginItemTooltip may react to the a widget in other windows. (I'm not sure whether I've misused the functions.)

Screenshots/Video:

(The title color is irrelevant here.)

Minimal, Complete and Verifiable Example code:

ImGui::Begin("Test");
{
    ImGui::Button("...", {100, 100});

    if (1) {
        ImGui::Begin("Strange");
        ImGui::End();

        // The same window, intentionally do not early out:
        // When this window is collapsed, `BeginItemTooltip` will react to the "..." button above.
        // (When not collapsed, will react to "123" button normally.)
        ImGui::Begin("Strange");
        ImGui::Button("123");
        if (ImGui::BeginItemTooltip()) {
            ImGui::Text("Hovered");
            ImGui::EndTooltip();
        }
        ImGui::End();
    } else {
        // A single block do not have the same problem.
        ImGui::Begin("Strange");
        ImGui::Button("123");
        if (ImGui::BeginItemTooltip()) {
            ImGui::Text("Hovered");
            ImGui::EndTooltip();
        }
        ImGui::End();
    }
}
ImGui::End();

[ocornut]

The problem is caused by the fact that on a Begin() call to append, we don't call this block:

// We fill last item data based on Title Bar/Tab, in order for IsItemHovered() and IsItemActive() to be usable after Begin().
// This is useful to allow creating context menus on title bar only, etc.
SetLastItemData(window->MoveId, g.CurrentItemFlags, IsMouseHoveringRect(title_bar_rect.Min, title_bar_rect.Max, false) ? ImGuiItemStatusFlags_HoveredRect : 0, title_bar_rect);

Therefore we inherit last value of g.LastItemData.ID & co.

There is special handling in IsItemHovered() to handle situation where a window is collapsed, leading all widgets (e.g. Button("123") to early out and therefore never write to this identifier.

We could perfectly compute title_bar_rect and copy the whole block in the else/appending case:

// When reusing window again multiple times a frame, just append content (don't need to setup again)
if (first_begin_of_the_frame)
{
    ....
}
else
{
   // Append
   SetCurrentWindow(window);

   ImRect title_bar_rect = window->TitleBarRect();
   SetLastItemData(window->MoveId, g.CurrentItemFlags, IsMouseHoveringRect(title_bar_rect.Min, title_bar_rect.Max, false) ? ImGuiItemStatusFlags_HoveredRect : 0, title_bar_rect);
}

That technically also fixes using e.g. IsItemHovered() right after an append-Begin():

ImGui::Begin("Another #7506 issue");
ImGui::End();
ImGui::Begin("Another #7506 issue");
if (ImGui::IsItemHovered())
    ImGui::SetTooltip("Hovering title bar!");
ImGui::End();

This is now fixed as well.

Actual fix goes through a function as the logic is longer in docking.
Pushed fa0120e

[ocornut]

Added a regression test: ocornut/imgui_test_engine@fc5105f