Docking: Trying to avoid an empty central node

[franciscod]

Version: 1.77 WIP @ 7ddc1ad
Branch: docking
Back-ends: imgui_impl_sdl_gl3.cpp
OS: Linux

I would like to not have an empty central node if there's other node adjacent to it. Instead when removing the last window docked into the central node, the closest dock node should fill the available space, and turn into the new central node.

Is there anything to be aware of before attempting to add this functionality?

I see ImGuiDockNodeFlags_NoCentralNode is commented since the line was introduced, so maybe there's a reason for that.

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DockContextProcessUndockNode has this in a comment:

// In the case of a root node or central node, the node will have to stay in place. 

Comment before ImGui::DockSpace's code:

// The Central Node is always displayed even when empty and shrink/extend according to the requested size of its neighbors.

[ocornut]

I believe this is same as #4334 so linking them together