.category-children-item {
  display: flex;
  float: none !important;
}

seems to do the trick.

EDIT:
after looking a bit at the topic @crazycells linked and looking at this for a few more minutes I think the better solution would be:

.categories .category-children {
  display: flex;
  flex-direction: column;
}

It doesn't affect how it works, but not using !important is the improvement here

Oh, and if you want some distance between the items just add margin-top or margin-bottom to .categories .category-children-item.

If you want two columns try this:

.categories .category-children {
  display: flex;
  flex-wrap: wrap;
  flex-direction: row;
}
.categories .category-children-item {
  flex: 1 1 45%;
  margin-top: 1vh;
}

You can increase the number of columns by changing that 45% to some lower number (it actually can be a little higher, but it seemed to go to 1 column on above 48% for me. 45% was just more "round" number close by
That margin-top is there because otherwise the items were touching at the top and bottom and it didn't look good for me

The only problem here is with longer subcategories - especially with mobile devices. I think if I wanted to do something like this I would try to change it to just one column if the screen width is too small.

@crazycells
Responding from a phone, so I can't make a full solution right now, but I think the best way would be to select it from correct <li> element using cid. Like this:

.categories li[data-cid=x] .category-children {
...
}
.categories li[data-cid=x] .category-children-item {
...
}

Where x should be replaced with the cid of the category you want to style.

I didn't check this, but it should work

You could also use :nth-child, but I think that's worse because reordering categories or adding subcategories to one that didn't have them might break it.

I was interested in that functionality as well, so I developed my own version of it.
It can be found here and already is in nbbpm (can be installed from ACP by searching for nodebb-plugin-category-queue)
It has some basic ACP (list of categories with options to queue new topics from them or not queue), and only works on new topics, not all posts (that is actually by design, as - after all - you have done the work on filtering posts already. It's just that I was looking for only sending new topics to queue)

I believe there are two systems for data flow from backend to frontend.

1. Templates. You can pass data to the renderer as a js object and fill the template with it. Use curly brackets to refer to it.
   Square brackets will give you a translated string.
   For more on templates look here

2. Websockets. It's not anything unique to NodeBB, so there should be no trouble finding documentation of socket.io
   Have an example of a plug-in using them: frontend and backend

Both systems work with widgets, so choose the option you prefer. The templates seem better if the data you want to load isn't supposed to change after the page loaded (there is lazy loading if you want to use it though), while with websockets you will have some delay between page load and data getting from backend to frontend, but you can request it dynamically without any reloads.

Edit: if you just need some more data about an user or something there is also the api - basically you get json data from almost any page you prepend /api/ to. For example community.nodebb.org/api/user/oplik0

I have the same problem...
For now I handled it by manually editing some NodeBB files:
in src/middleware/user.js in function middleware.checkAccountPermissions I changed line 135:

// For the account/info page only, allow plain moderators through
if (/user\/.+\/info$/.test(req.path)) {
	user.isModeratorOfAnyCategory(req.uid, next);
}

to :

// For the account/info page only, allow adminstrators and global moderators through
if (/user\/.+\/info$/.test(req.path)) {
	user.isAdminOrGlobalMod(req.uid, next);
}

That should limit user info page to only administrators and global moderators, but just to be sure I also (err... Actually, that's what I did first, only later noticing the possible change in middleware) modified src/controllers/accounts/info.js changing all isAdminOrGlobalModeratorOrModerator to isAdminOrGlobalModerator - that actually just makes the page not render for category moderators, causing 504 error after some time. The middleware change actually makes the website display a proper "access denied" page

As for seeing user IPs in posts, I had to modify src/socket.io/tools.js. More specifically, line 76:

if (!results.isAdmin && !results.isGlobalMod && !results.isModerator) {
	posts.ip = undefined;
}

had to be changed to:

if (!results.isAdmin && !results.isGlobalMod) {
	posts.ip = undefined;
}

And now only admins and global mods get to see the IP in post tools

Now, there is one more problem I didn't yet manage to find a solution to: if you are a moderator, you can see recent ips on users profiles in the source of the page. They are hidden with display: none, so you have to actually look at the source, but still - it would be better if this was visible only to administrators and global moderators :V I haven't found the code responsible for adding this invisible element, so I wasn't able to change it.

Oh, and obviously this probably isn't the best method of changing things, as you are modifying your local NodeBB installation and I have no idea what will happen with updates. But it works, and I don't see a better way of doing that right now.

A small plugin that allows you to join a group with the same name as a category directly from the category view.

Some screenshots:

NPM
GitHub

Manual installation:

$ npm install nodebb-plugin-category-join-group

@yariplus Already done that and even added one of these features myself
After I finish with some more important plugins for my use case I will probably try to add the second thing I wrote about in my issue there - that is deleting categories (although now I think a better option is only letting users disable them, so they can be restored by an administrator).

It is actually planned:
https://github.com/NodeBB/NodeBB/issues/5736
Two of the permissions that are supposed to be split are Global account info access and
Global user sessions access.
Unfortunately, there seems to be no ETA, and the issue is from 2017, so it might take some time before it's added... But there is hope

pitaj created this issue in NodeBB/NodeBB

open Privileges refactor #5736

You can enable the post queue for new users in post settings, but if you want to queue everything in one category you have to turn to plugins.
I actually created one that might help a little: https://github.com/oplik0/nodebb-plugin-category-queue
But because it was created just for my needs it only queues topics and not every new post (if you post something in a topic from that category it won't be queued, but if you create a new topic it will).
It can be easily modified to do that tho (look here for explanation on how to get cid from such a post).

Are you using a non-default skin? I noticed that most of them are broken here.

I believe there are two systems for data flow from backend to frontend.

1. Templates. You can pass data to the renderer as a js object and fill the template with it. Use curly brackets to refer to it.
   Square brackets will give you a translated string.
   For more on templates look here

2. Websockets. It's not anything unique to NodeBB, so there should be no trouble finding documentation of socket.io
   Have an example of a plug-in using them: frontend and backend

Both systems work with widgets, so choose the option you prefer. The templates seem better if the data you want to load isn't supposed to change after the page loaded (there is lazy loading if you want to use it though), while with websockets you will have some delay between page load and data getting from backend to frontend, but you can request it dynamically without any reloads.

Edit: if you just need some more data about an user or something there is also the api - basically you get json data from almost any page you prepend /api/ to. For example community.nodebb.org/api/user/oplik0

Seems like it can work without any backend, although is easy to circumvent when used in that way. I think the hardest part here would be making it look sensible, as the version on that website seems far too big for the composer
As for implementing it properly - you'd have to somehow pass its response along with the post, but if you manage to do that it shouldn't be hard to actually handle it as it's just one simple http request to their api.

Not by default, but it would be possible to make a plugin that allows for it.

There is filter:post.shouldQueue, that sends everything just before a post is added. I think data passed there contains the post contents, so a plugin could just check for these things and decide to queue only if post contains one of them.

As you want it to still apply only to guest/new users, a better option would probably be to set the queue for them in settings and let plugin refuse to queue when it doesn't find anything from your list of filtered things.

There is search in the topic.
In user settings go to default user settings section and there should be an option to turn it on.
Each user can also change that setting in his user settings (there is a section with search settings)
It's not ideal, as it doesn't highlight the searched word and just takes you to the post it was found.

And as PitaJ said - you can improve the search, but you need to change the search engine from standard db search - as it just can't do what you want it to.
There is a solr plugin and unfortunately unmaintained elasticsearch one (So it might not work anymore). But I think you need to install solr and elasticsearch first to actually provide them with their backend.

If there is some light search engine better than database internal search, perhaps making a plugin with it and installing it by default would be better. But I didn't really look into internal search engines so no idea if that's possible or a good idea for default install.

EDIT: after a quick google search I found lunr.js (and elasticlunr that is supposed to be just lunr with some more functions), and flexsearch as node-based alternatives to solr. No idea if they make sense for this application, but I might try looking into a plugin using one of them after I finish with some other projects (so probably a month from now or more).

@KarlPro123 Yeah, just make sure you don't have them turned off in post settings and use markdown to add an image there: ![alt-text](image-url) (you can skip the alt-text obviously)

@yariplus Already done that and even added one of these features myself
After I finish with some more important plugins for my use case I will probably try to add the second thing I wrote about in my issue there - that is deleting categories (although now I think a better option is only letting users disable them, so they can be restored by an administrator).

Some of these points are basically "make NodeBB into something else than a forum software". More specifically, I'm talking about c, d, e, i and j.

And I would say a lot of them aren't really fit for core NodeBB, but rather for plugins:
f. Are they really empty? The only thing Quora has that NodeBB doesn't is this right bar with credentials and "knows about". And I wouldn't say adding this to the profile is best for core NodeBB - it seems more like a plugin material. Oh and look, there is at least one plugin for this: https://github.com/NicolasSiver/nodebb-plugin-ns-custom-fields
And there are a few fields that NodeBB does have and Quora doesn't: reputation, website, when user joined the site and when he was last online.
So I wouldn't say they are empty, and you can always use plugins to populate them more

c. There is this plugin https://github.com/yariplus/nodebb-plugin-modmin (you can let everyone create their own category and manage it), but I think it would need a little more work to be really good at what you want

e. I would also say that this isn't really core NodeBB material as newsfeeds aren't really a forum thing, but still - a plugin adding them would be great. And I'm just an user not a maintainer, so maybe someone from NodeBB disagrees and it will make it way there some day

g. I think all monetization is left to plugins. Even your standard adsense is only a plugin.

i and j - as I said, they are basically "make NodeBB less like forum and more like some other website type", but I think plugins and themes that allow you to do this would be nice. There are some plugins for question and answer type of topics, but nested replies might be nice Not sure if that is currently possible to do though, so maybe allowing that kind of customization might be a good change.

k. Not sure what do you mean here - you can get a link to any post from its timestamp (just right click it and select copy link location, or whatever your browser calls this option). For example, the link to your post here is https://community.nodebb.org/post/75846
If you mean linking to topics/posts by specific users, take a look at user profiles In the hamburger menu you will find your topics, posts, and best posts (by reputation they got)

I don't have anything against a, b, d, h, l, m and n, as they are mostly just improvements.
AMP topics would be great, but I'm not sure if it's as simple to implement as you're saying here. NodeBB has a lot of js everywhere and I'm not sure if amp elements could replace it all. So it would probably not just be "turn custom js to AMP elements", but rather "rewrite topic pages entirely for AMP". But I never did anything with AMP, so I might be wrong

I'm also hoping for official PWA support, as right now you can only get a basic one (no notifications) by modifying a core NodeBB file (src/controllers/index.js) and your nginx config (explained here: https://community.nodebb.org/post/59669). In the next NodeBB version I think this will be possible with a plugin (I added a filter:manifest.build as was suggested in the next post in that topic), but still - as NodeBB is striving to be most progressive a better manifest and some default service worker (with correct front end js in place) in core would be nice.

There is this plugin: https://github.com/yariplus/nodebb-plugin-modmin

Basically does what you want, allowing you to give someone a permission to manage a specific category - including creating subcategories there.

@crazycells
Responding from a phone, so I can't make a full solution right now, but I think the best way would be to select it from correct <li> element using cid. Like this:

.categories li[data-cid=x] .category-children {
...
}
.categories li[data-cid=x] .category-children-item {
...
}

Where x should be replaced with the cid of the category you want to style.

I didn't check this, but it should work

You could also use :nth-child, but I think that's worse because reordering categories or adding subcategories to one that didn't have them might break it.