Question: what's wrong with ExtJS. Granted, I haven't used it in several years and at the time found that it produced a lot of difficult to track down bugs, but at the same time it seemed like a reasonable choice for a certain type of app. If you need to build something that emulated a desktop app in the browser, say for an internal dashboard, why would Ext be a bad choice today?
The last time I used ExtJS was a few years ago, so it may have changed but: Its tables all the way down. Horrible html generated from a really clunky declarative library. If you only ever _just_ want to make a windows 2000 deskop-like experience for older browsers, than yes it is serviceable. Also very painful to extend / create custom work when you're dealing with that level of nesting. ExtJS solves an old problem that you very likely will not run in to today. Better to use modern-ish libraries and use SASS or LESS to get that good 'ol early 2000s desktop look.
ExtJS has two modes, modern mode is based on sencha touch and uses CSS-based layout, classic mode is based on the older ExtJS rendering model and uses tables for layout. You get a richer feature set in classic mode, but the end goal is to achieve parity and then phase it out.
They're a pretty isolated eco system, although it seems they're working to improve that. Right now they use their own ant-based build system, have their own ES6-incompatible class system and you're pretty far removed from HTML/CSS. So it's really hard to leverage "regular" JS frontend skills there.
Which might be fine if the system would work properly, as Ext would not directly compete with more "bare metal" component systems, but work as some kind of browser-based GUI DSL (which was what got a lot of people into it back in the days when DHTML was young).
Sadly, they seem more focused on introducing new features than fixing bugs or making all those elements work together (recent example: Conflict between new two-way binding and old statefulness of grid column sorting/order/filtering).
Mindshare isn't as good as with almost any other lib (some guy had the same problem you have with 6.2 with version 3.4, no solution ever posted), and due to it not being open source, there's not exactly a large community working on it, fixing things, adding new components etc.
And for all that, you have to pay a pretty stiff fee.
