Dear ImGui users: I've finally reworked BeginChild() api to switch that annoying 'bool border = false' param to becomes flags (miraculously not API breaking!), and added various new features to child windows in the process, including manual resize, auto-resize etc. BeginChild("name", .., ImGuiChildFlags_ResizeX);
Double-click to auto-fit on single axis (also works on regular windows). Saved in settings.
Also see: using auto-resize on Y axis + a constraint so it grows up to a maximum number of lines, convenient idiom.
There are various ways to achieve similar things but this one is quite accessible now.