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I'm not sure if this is what you're looking for, but Dear ImGUI is kind of a gold standard within the AAA game development industry. It's MIT licensed and u/ocornut is one of the most helpful and coolest dudes in the open source world.
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InfluxDB
InfluxDB – Built for High-Performance Time Series Workloads. InfluxDB 3 OSS is now GA. Transform, enrich, and act on time series data directly in the database. Automate critical tasks and eliminate the need to move data externally. Download now.
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I haven't actually used either one, but there's HikoGUI (previously known as TTauri) under the Boost license and RmlUI under MIT. Not sure if they're very OS-integrated in the way you want or not.
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Elements was mentioned as a specific example of an MIT-licensed GUI library on the CPPcast episode from 5 May 2022.
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https://github.com/cnjinhao/nana Boost Software License
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LVGL (https://lvgl.io/) - MIT
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I haven't actually used either one, but there's HikoGUI (previously known as TTauri) under the Boost license and RmlUI under MIT. Not sure if they're very OS-integrated in the way you want or not.
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There is NAppGUI: https://github.com/frang75/nappgui_src. It’s written in C, but I’m sure you could wrap the API.
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SaaSHub
SaaSHub - Software Alternatives and Reviews. SaaSHub helps you find the best software and product alternatives
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GacUI
GPU Accelerated C++ User Interface, with WYSIWYG developing tools, XML supports, built-in data binding and MVVM features.
You can try https://github.com/vczh-libraries/GacUI . It uses XML descripted UIs, which you may be familiar with as a C# developer.
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ares
ares is a cross-platform, open source, multi-system emulator, focusing on accuracy and preservation.
For something a bit more unusual, there's hiro. It depends on nall and I think ruby (both also in the same repo), it's not entirely native-looking especially in a post-Win32 world - though I believe most of the replacements like the file dialog were optional - and it was never properly documented before the original author died; but it's ISC-licensed, performant for end-users, and the API is clean and straight-forward.
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ultimatepp
U++ is a C++ cross-platform rapid application development framework focused on programmer's productivity. It includes a set of libraries (GUI, SQL, Network etc.), and integrated development environment (TheIDE).
https://www.ultimatepp.org/ is BSD/MIT.
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nanogui
Minimalistic C++/Python GUI library for OpenGL, GLES2/3, Metal, and WebAssembly/WebGL (by mitsuba-renderer)
Nanogui : https://github.com/mitsuba-renderer/nanogui
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