High-ho it's another website rebuild. First, a question regarding sidebars

Okay, my site has developed a serious glitch which seems to be related to my using way too many dropdown menus. Or at any rate that's where things started glitching first. I knew that the site was a stop-gap when I uploaded it, so I'm not too surprised to be back at square #1 again. I'm not a web developer and don't especially enjoy it, but I've had the site for 20 years, and don't feel like giving it up. However, at present it's next to unusable.

It's a largish site with something on the order of a couple of hundred pages. So it needs a lot of navigation assistance. It is split up into a number of different collections and sub-collections, but a couple of those contain up to something like 80+ pages, and thats a lot of links.  

The current site uses dropdowns from a fixed-position header. But that doesn't seem to be working out at all. It wasn't *quite* as bad before I split the menus up into many more dropdowns to shorten the menus, keeping fully visible on a laptop screen. But it was already glitching.

The previous iteration of the site used a classic left sidebar with fly-out menus. That site was built in CS6 which included Spry assets and it worked very well for most of a decade. Couldn't handle small screens, because the fly-outs extended beyond what the screen would show, and they weren't exactly scrollable.

I'd really prefer not to have to build a permanent sidebar, since that takes up additional screen real estate. I have been browsing the W3 Schools site, and have seen a number of things that might be helpful, if they worked. Unfortunately, attempting to insert their W3 proprietary code into a test template hasn't been going well.

One example, which looked promising was a sidebar which could be invoked by clicking on a button, and which went away by clicking a 'close' button on the sidebar. I was not able to get it to work in my Dreamweaver CC2021 test site. This was one of the propritary W3.css examples. 

So. Does plain vanila .css, such as I know is used in a Dreamweaver site, do anything like that? Which is to say, invoke a panel, overlaying part of a page containing navigation buttons, and will go away by clicking a 'close' command?

It wouldn't necessarily have to be a sidebar. It could come in from any direction that's visible onscreen. But it shouldn't cover the whole page. At least part of the basic page should be visible, so it's obvious that we've not just moved to a different page. 

So, can anyone point me at some possibilities regarding sidebars? I'll be posting some other questions about some other navigation methods. But having been bitten by dropdown menus, I'm not especially eager to try another attempt at using those. Or, not unless they could be opened with a click, and *stay* open until clicked closed, so one could scroll down to see the whole thing. I do know that W3.css has some code for doing those. But, like I say, I'm not having a lt of luck in incorporating their code into my base pages.