Assert in BeginDragDropSource when dragging window to bottom of screen

[aclysma]

Version/Branch of Dear ImGui:

Version: 1.84.1 (58f5092)
Branch: docking

Back-end/Renderer/Compiler/OS

Back-ends: SDL/OpenGL3 (Modified the example_sdl_opengl3 example)
Operating System: MacOS

My Issue/Question:

I am getting this fatal assert: Assertion failed: (0), function BeginDragDropSource, file ../../imgui.cpp, line 10561.

Steps:

1. Modify example_sdl_opengl3
   a. Removed io.ConfigFlags |= ImGuiConfigFlags_ViewportsEnable;
   b. Removed the existing gui (everything between ImGui::NewFrame(); and ImGui::Render();)
   c. Added the below code and called it between ImGui::NewFrame(); and ImGui::Render();
2. Drag the window past the bottom of the screen

I'm not trying to use multiple viewports/create a window outside the window (I disabled ImGuiConfigFlags_ViewportsEnable). It just happens that if you drag a floating window down there with the below code, it will fatal assert.

Screenshots/Video

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

void test_app() {
    if (ImGui::Begin("ExtraWindow")) {

        // Does not occur without this BeginChild/EndChild call
        ImGui::BeginChild("##Test", ImGui::GetContentRegionAvail(), false);

        // Does not occur if loop count <= 1
        for (int i = 0; i < 2; ++i) {
            // Does not occur without the BeginGroup/EndGroup call
            ImGui::PushID(1 + i);
            ImGui::BeginGroup();

            ImGui::Button("item");
            if (ImGui::BeginDragDropSource()) {
                int x = 0;
                ImGui::SetDragDropPayload("ITEM_TYPE", &x, sizeof(int), 0);
                ImGui::EndDragDropSource();
            }

            ImGui::EndGroup();
            ImGui::PopID();
        }

        ImGui::EndChild();
    }

    ImGui::End();
}

Other Notes

The comments around the assert are here. I read the comments and the code below it. My take from reading it was that as long as the widget has a unique ID, I should be fine. But I'm not certain I understand what's being said.

// If you want to use BeginDragDropSource() on an item with no unique identifier for interaction, such as Text() or Image(), you need to:
// A) Read the explanation below, B) Use the ImGuiDragDropFlags_SourceAllowNullID flag, C) Swallow your programmer pride.
if (!(flags & ImGuiDragDropFlags_SourceAllowNullID))
{
    IM_ASSERT(0);
    return false;
}

I also tried changing the button to:

char id[128];
sprintf(id, "item##%d", i);
ImGui::Button(id);

But I was expecting by pushing the ID, that this would not be necessary. (But really not sure)

Adding the flag to the call (i.e. ImGui::BeginDragDropSource(ImGuiDragDropFlags_SourceAllowNullID)) skips the assert, but I'm not sure what the downside of this is. I thought, especially with the change to sprintf a unique ID, that I would not need this.

Paste from About ImGui

Dear ImGui 1.84.1 (18403)
--------------------------------
sizeof(size_t): 8, sizeof(ImDrawIdx): 2, sizeof(ImDrawVert): 20
define: __cplusplus=199711
define: __APPLE__
define: __GNUC__=4
define: __clang_version__=12.0.0 (clang-1200.0.32.29)
define: IMGUI_HAS_VIEWPORT
define: IMGUI_HAS_DOCK
--------------------------------
io.BackendPlatformName: imgui_impl_sdl
io.BackendRendererName: imgui_impl_opengl3
io.ConfigFlags: 0x00000041
 NavEnableKeyboard
 DockingEnable
io.ConfigViewportsNoDecoration
io.ConfigMacOSXBehaviors
io.ConfigInputTextCursorBlink
io.ConfigWindowsResizeFromEdges
io.ConfigMemoryCompactTimer = 60.0
io.BackendFlags: 0x0000140E
 HasMouseCursors
 HasSetMousePos
 PlatformHasViewports
 RendererHasVtxOffset
 RendererHasViewports
--------------------------------
io.Fonts: 1 fonts, Flags: 0x00000000, TexSize: 512,64
io.DisplaySize: 1280.00,720.00
io.DisplayFramebufferScale: 1.00,1.00
--------------------------------
style.WindowPadding: 8.00,8.00
style.WindowBorderSize: 1.00
style.FramePadding: 4.00,3.00
style.FrameRounding: 0.00
style.FrameBorderSize: 0.00
style.ItemSpacing: 8.00,4.00
style.ItemInnerSpacing: 4.00,4.00

[ocornut]

Interesting, the issue is because the child window is entirely out of view (BeginChild() returned false): all items are now coarse clipped early on so e.g. the Button() is not submitted and LastItemID is always zero.

The workaround/fix is to enclose your child contents submission under a if (BeginChild()) { } block (also generally better).

As per specs we should fix it tho.

[ocornut]

The assert is misleading here: it is meant for the actual case of using items without ID as a drag source, e.g. a Text() element. In this case because of the clipping the assert erroneously triggers.

Fix coming soon.

[ocornut]

Pushed a fix now! Thanks for the report and careful repro (we'll be adding some regression tests based on it).

A similar issue would manifest with either BeginDragDropSource() or BeginDragDropTarget() in docking branch when submitting items in a Begin() returning false (by dragging e.g. over the tab for that window).

[aclysma]

Thanks! BTW, do you think BeginDragDropTargetCustom should get the same treatment as BeginDragDropTarget? I have no idea, just noticed the code is very similar.

[ocornut]

do you think BeginDragDropTargetCustom should get the same treatment as BeginDragDropTarget

It already had that check in.