Docking a Window manually causes an assert indised of DockNodeFindInfo

[Link459]

Version/Branch of Dear ImGui:

Version: 1.91.5
Branch: docking

Back-ends:

imgui_impl_vulkan.cpp + imgui_impl_sdl3.cpp

Compiler, OS:

linux + gcc 14.2.1

Full config/build information:

[Window][Debug##Default]
Pos=60,60
Size=400,400
Collapsed=0

[Window][Hello, world!]
Pos=283,408
Size=339,180
Collapsed=0

[Window][Dear ImGui Demo]
Pos=899,214
Size=550,680
Collapsed=0

[Window][WindowOverViewport_11111111]
Pos=0,0
Size=1916,1076
Collapsed=0

[Window][Viewport]
Pos=0,0
Size=1916,1076
Collapsed=0
DockId=0x03954D96,0

[Window][Another Window]
Pos=60,60
Size=198,71
Collapsed=0

[Docking][Data]
DockSpace ID=0x03954D96 Window=0xA87D555D Pos=0,0 Size=1916,1076 CentralNode=1 Selected=0x13926F0B

Details:

Docking window results in an assert inside DockNodeFindInfo

I was trying to programmatically attach a window to a dock space on the central node.
It was attaching the window correctly as far as I can tell but once I tried to manually attach another window to the dockspace I got this assert:

void DockNodeFindInfo(ImGuiDockNode*, ImGuiDockNodeTreeInfo*): Assertion `node->IsLeafNode() && "If you get this assert: please submit .ini file + repro of actions leading to this."' failed.

This happened whenever I tried to attach a window to the left, right, bottom or top of the dockspace.

Screenshots/Video:

imgui_docking.mp4

Minimal, Complete and Verifiable Example code:

ImGuiID id = ImGui::GetID("dock_space");
ImGuiViewport* viewport = ImGui::GetMainViewport();
ImGuiID dockspace = ImGui::DockSpaceOverViewport(id, viewport,ImGuiDockNodeFlags_PassthruCentralNode | ImGuiDockNodeFlags_CentralNode);
if (ImGui::Begin("Viewport", nullptr,
    ImGuiWindowFlags_NoDecoration | ImGuiWindowFlags_NoCollapse | ImGuiWindowFlags_NoBackground | ImGuiWindowFlags_NoNavInputs | ImGuiWindowFlags_NoTitleBar))
{
    ImGui::End(); 
}
ImGuiID dockspace_id = ImGui::GetID("dock_space");
ImGui::DockBuilderDockWindow("Viewport", dockspace_id);
ImGui::DockBuilderFinish(dockspace_id);

[ocornut]

(
For the record, your code is a bit confusing because you are holding the same value in three spots:

ImGuiID id = ImGui::GetID("dock_space");
[...]
ImGuiID dockspace = ImGui::DockSpaceOverViewport(id, ....); // this return the value you passed to it (if you pass 0 it generates an id)
[...]
ImGuiID dockspace_id = ImGui::GetID("dock_space");

Therefore in your code id and dockspace and dockspace_id are all the same value!
)

I will investigate the main issue later.

[ocornut]

I was a little confused by this, and then realized the (non-obvious) issue:

Passing ImGuiDockNodeFlags_CentralNode to DockSpace() or DockSpaceOverViewport() is not only unnecessary (because DockSpace currently adds it) but it creates a huge bug as the flag is stored in node->SharedFlags and then not moved to inheriting child when split, creating a largely inconsistent state.

If you remove the ImGuiDockNodeFlags_CentralNode value it should work.
Note that in your repro you are calling DockBuilderDockWindow() every frame which is likely not desirable. You may want to check earlier if the node existed, or if you are in a special reset state, in order to call.

I am going to research if I can better document/fix this, possibly by making it clearly illegal to pass this flag to DockSpace(), but it's part of a larger required rework as the concept of CentralNode is quite confusing/broken right now.