I have pushed an experimental 'docking' branch:
https://github.com/ocornut/imgui/tree/docking
Effectively providing a long awaited official solution to the features discussed in #351 and #261.

TL;DR; You can benefit from good portion of docking features with no extra work or API call. Windows can be merged with each others, with or without partitioning the space.

Prefer creating separate New Issues for specific questions/issues, so they can more easily be closed when solved and not clutter this thread (which is already too big)

Please do not report issues without reading the 'Feedback' section of the next post and without reading the 'Contributing' document listed in there. The number of ambiguous, ill-formed questions and incomplete bug reports posted on this entire github is difficult to deal with. Thank you.

GIF (this is actually a gif from an older version which has some glitches, but you get the gist!)

Quick demo

You may checkout docking and build one the example e.g. example_win32_dx11/ , example_glfw_opengl3, example_sdl_vulkan to test this. Head to Demo>Configuration to see the global configuration flags. Note that multi-viewports are enabled by default in those demo but only well tested under Windows. If you have issues with them you can disable them via the Demo>Configuration pane or by commenting out the line that sets ImGuiConfigFlags_ViewportsEnable in the main.cpp of your example. Docking is functional without multi-viewports enabled.