Not sure how the api is gonna be for the user, maybe the user can use SameLine and render the resizable panel first and then another panel that fills the remaining part.
Having looked at the Dear ImGui source code, it just uses a left child window with a resize flag on it, which consumes up space on the left. This functionality could be added to regular groups.
Resizable Panel (Splitter)
A resizable panel similar to the one in ImGui
Not sure how the api is gonna be for the user, maybe the user can use
SameLine
and render the resizable panel first and then another panel that fills the remaining part.