A quick summary of the results: with ~300 answers, ~60% for the split
layout (public headers in include/, sources and private headers in
src/) and ~40% for the combined (everything in a single directory).

Split layout pros:

  - Public headers/API clearly separated and easy to identify for users,
    reviewers, and tools.
    
Split layout cons:

  - Hard to navigate between headers and sources in editors, GitHub, etc.

  - Split source code generation could be problematic.