Ask HN: Best CSS layout and widget set combination for desktop style web apps?

I'm leading a small team building out a suite of web apps, in many of which aspects of the design will mimic traditional desktop app UI patterns - independent scrollable areas, fixed footers, toolbars, complex layouts, etc.

We've built a couple of the more simpler apps using Bootstrap, and while the 'widget' set that the library offers is nice, it doesn't seem like this will offer the layout support that we are going to need in the more complex apps, or will it?

I've looked at Foundation for Apps and that seems promising. I've tried to get Flexbox to play nice with Bootstrap and failed. Grid style sheets looks kinda amazing. I've considered also just going pure Flexbox and rolling our own styling for UI components.

So, what combination of CSS layout approaches and reusable, styled UI widget sets have others had success with for developing complex web apps?

The rest of our front-end tech stack is React/webpack/ES6 (via Babel)

Responsiveness isn't necessarily a requirement but would be a nice bonus. The browsers that we do need to support are IE11+, FF34+, Chrome41+.