alrighty, thank you. i switched to Intel and dicked around in my BIOS but couldn't find a way to disable Nvidia.

what does this "failed nouveau" mean?
please post the output of

yes, of course.

00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 0b)
Subsystem: Acer Incorporated [ALI] Haswell-ULT Integrated Graphics Controller
Kernel driver in use: i915
Kernel modules: i915
00:03.0 Audio device: Intel Corporation Haswell-ULT HD Audio Controller (rev 0b)
Subsystem: Acer Incorporated [ALI] Haswell-ULT HD Audio Controller
--
03:00.0 3D controller: NVIDIA Corporation GM108M [GeForce 840M] (rev ff)
Kernel modules: nvidiafb, nouveau, nvidia_375_drm, nvidia_375

NO kernel driver is in use for the nvidia card.
you need to fix that.
not saying it will help with those invisible windows though.

did you learn that because there's no "Kernel driver in use" line between the 3D controller Nvidia line and the kernel modules line?
i'm guessing this doesn't have anything to do with switching to Intel as my active graphics card.

i tried searching for how to put one into use for it but i'm very unsure i found what i was looking for. do i just assign one of the kernel modules to it? if so, does which one matter? (also, uh, how do i do that? i only found a command to see if the Nvidia driver is in use, and one for finding out about the details of it:
is that helpful to me at all? i mean... do i need to be such a pussy about putting commands into my terminal? i'm worried i'll "break" something, but rather than break that i'll change something i then won't be able to clearly identify while wanting it undone. although, mostly, when entering ill-thought out commands, i just got error msgs and i can live with those. (lol i'm two weeks in and already fucked up a lot. but i guess such is the process of learning))

ok, i didn't take that into account.
truth be told i know nothing about hybrid graphics.
one thing i forgot to ask is, what window is almost invisible, which app is it? is it always the same one?

since all this is not really going anywhere, you should try to

• adjust your window managers settings somehow
• try a different window manager
• try a different DE
• try a different distro

that's a GREAT question. it's not always the same one, i had it happen again just now without opening anything anew, but by minimizing and trying to maximize my browser again. (btw i added some screenshots to the original post, for further/easier understanding)
But I realized that without fail, this happens when I am on the bare desktop (=when i'm facing my wallpaper). it never happens when i have sth open and want to open another thing, I THINK. i've only now noticed the connection.

anyway, thanks a ton, i'll go with your suggestions, leave Cinnamon and see where that takes me.

re post #1: that's clearly a browser window with youtube.com opened.

troubleshooting:

• see if your browser is running even after you closed it completely (chromium-based browser do that) => 'pidof chromium' or some such
• try a different browser
• try a different web page (no video, e.g. this page)

uh, how come knowing i was on yt changed anything?

^ because the flash/html5 video might be using hardware accelaration, which brings us back to the non-working nvidia card etc. etc.

edit:
look, there's no hand holding step by step walkthrough for this situation.
you have to roll up your sleeves and get your hands dirty.

legit not what i'm asking for.

if we're back to Nvidia not working, then Intel isn't working either, since it happened with both. that's why i ruled it out. i've meanwhile switched DEs, so far it hasn't happened in xfce. if it does, i'll turn off compositing. if it does again, i'll switch distros. if it does again, well then i'll probably attempt to become a hybrid graphics expert. i don't currently have the time* to look into all the details, and no, i'm not asking you to, either. right now i'm putting it off since for the foreseeable future/circumstances, i have a game plan.

i'm not asking anything of you right now.
but while we're at it, thank you for all the help you've given me.

(*edit: i say i don't have the time because i've already put a lot of time into this. several hours in some instances; searching the web, forums, wiki and similar articles. it didn't help as much as it should have, probably because i'm embarrassingly new to everything. which is not an excuse, just an explanation. i've already gotten my hands dirty.)

^ 'k, fair enough.

well at least for xfce i know that this is possible, via settings gui.

I KNOW! i checked before changing to it. ;)

With a wide screen display such as yours, I like to use XFCE4's excellent implementation of vertical panel. As far as I'm aware so far, it's the only DE/WM which rotates text so I can have text labels on the window buttons.

Honestly, this is the "killer feature" which keeps me on XFCE4. Everything else (that I know of so far) only does horizontal text on window buttons, and I hate losing vertical screen space on the taskbar.

duuude. hella cool.

i just read that they're working on such an update/feature for cinnamon :) if that's of interest to you