Meanwhile, open-source, non-Electron, multi-platform software that handles copy-paste via system clipboard exists just fine (VCV Rack comes to mind).
There are many things it makes sense to copy/drag-drop between applications. There are cases where you might not need to, but using the system clipboard is still common.
> Meanwhile, open-source, non-Electron, multi-platform software that handles copy-paste via system clipboard exists just fine (VCV Rack comes to mind)
VCV Rack seems to just use GLFW to handle it, but again I'm not claiming these issues are unavoidable or more likely than not to occur. For any given issue with some software, 99% of other software will not have that same issue.
They're sufficient in many cases, but you'll still sometimes need the control of working with COM/etc. directly, and those libraries don't fully save you from platform-specific bugs (e.g: https://github.com/glfw/glfw/issues/2644).
> the app in question has no use for system clipboard in the first place
What do you expect to happen when you copy some text from an external editor into a text field?
A hybrid clipboard system is possible, but adds complexity and won't necessarily eliminate platform-specific bugs.
> No app needs to worry about copying text between text fields, system APIs take care of that
Platform-specific APIs that need to be called using platform-specific implementations, and thus are a reasonable cause of platform-specific bugs.