I think the classic OSS community will be a bad starting point for this just for this reason, you either start by booting off GTK/QT and end up with a bloated monstrosity that many hardcore C++ developers from other domains will bypass or start from scratch and struggle with basics like text-rendering that you mention.

As someone coming from gamedev, I'd prefer something like this to be few files (maybe even single-file-header lib) that mostly wraps win32,osX,etc core API's for a native experience with good accessibility,etc support. My declarative-style experiment was maybe 6-7 files medium sized files (and I think the implementation ran away with C++ bloat and complexity trying to get the API to mesh updates with C++)