SDL backend: Drag&drop with floating windows doesn't work

[DimaKoltun]

---

Version/Branch of Dear ImGui:

Version: 1.88 WIP
Branch: docking

Back-end/Renderer/Compiler/OS

Back-ends: imgui_impl_sdl.cpp + imgui_impl_opengl3.cpp
Compiler: MSVC
Operating System: Windows 10

My Issue/Question:

If we drag item from from floating window that isn't fully located under main window, then drop to another window doesn't work

Screenshots/Video

2022-09-26.14-59-54.mp4

Standalone, minimal, complete and verifiable example: (see #2261)

        ImGui::Begin("Asset Browser");
        if (ImGui::Button("Drag this"))
        {

        }
        int dndData = 1;
        if (ImGui::BeginDragDropSource(ImGuiDragDropFlags_None))
        {
            ImGui::SetDragDropPayload("DND_TEST", &dndData, sizeof(int));
            ImGui::EndDragDropSource();
        }
        ImGui::End();

        ImGui::Begin("Viewport");
        ImGui::Text("Dropped count: %d", droppedCount);
        if (ImGui::Button("Drop Here"))
        {

        }
        if (ImGui::BeginDragDropTarget())
        {
            if (const ImGuiPayload* payload = ImGui::AcceptDragDropPayload("DND_TEST"))
            {
                droppedCount += *(const int*)payload->Data;
            }

            ImGui::EndDragDropTarget();
        }
        ImGui::End();

[ocornut]

It appears to be a bug in the SDL backend.
We added relaxed suppot for io.BackendFlags |= ImGuiBackendFlags_HasMouseHoveredViewport; in 1338eb3, however the Hovered Viewport reported by SDL in this situation is wrong.

A temporary workaround you can use is to call io.BackendFlags &= ~ImGuiBackendFlags_HasMouseHoveredViewport; after ImGui_ImplSDL2_InitXXX(). Will investigate further.

[ocornut]

I experimented and couldn't get SDL to send any SDL_WINDOWEVENT event for the underlying window, even when disabling the SDL_CaptureMouse() flag.

I narrowed it down to a new feature of SDL 2.0.22:
https://wiki.libsdl.org/SDL_CaptureMouse

Please note that as of SDL 2.0.22, SDL will attempt to "auto capture" the mouse while the user is pressing a button; this is to try and make mouse behavior more consistent between platforms, and deal with the common case of a user dragging the mouse outside of the window. This means that if you are calling SDL_CaptureMouse() only to deal with this situation, you no longer have to (although it is safe to do so). If this causes problems for your app, you can disable auto capture by setting the SDL_HINT_MOUSE_AUTO_CAPTURE hint to zero.

I am pushing a fix to the backend now, need to call SDL_SetHint(SDL_HINT_MOUSE_AUTO_CAPTURE, "0");

[ocornut]

The full fix is 51bbc70 and has been merged in docking.
Thanks for the careful bug report and repro.

[ocornut]

Previous fixes caused issue, pushed a new fix d7aca16, see #5012 for details.