http://www.cmyr.net/blog/gui-framework-ingredients.html has a bunch of what you'll need to know about building a GUI from scratch, especially if you're interested in any of the "hard parts" rather than making a toy. It's Rust rather than C++ but many of the principles are similar.
Other than that, I do recommend the immediate mode GUI approach if you're writing everything yourself, as it's the lowest overall system complexity of all the approaches. It has a number of drawbacks, and things don't abstract/compose as easily as a standard object-oriented retained, or a newer React/SwiftUI approach, but exposing the raw parts has its upsides.
Other than that, I do recommend the immediate mode GUI approach if you're writing everything yourself, as it's the lowest overall system complexity of all the approaches. It has a number of drawbacks, and things don't abstract/compose as easily as a standard object-oriented retained, or a newer React/SwiftUI approach, but exposing the raw parts has its upsides.