@rgplayzmc **Update To Previous Post **
Solution: Until @xZero updates his script (It returns

opera-fix-ffmpeg.sh: 15: [: Illegal number: 
Error: snapd is not installed
Command 'snap' not found
/usr/bin/snap

Snap is actually installled
)
Here's what to do (GUI method):

1. Install the chromium-ffmpeg snap
2. Navigate to /snap/chromium-ffmpeg/current
3. Find the folder with the highest number and open terminal over there (Right click->open with terminal).
4. Run

sudo cp libffmpeg.so /usr/lib/x86_64-linux-gnu/opera

(Replace opera directory with wherever yours is installed.)

(CUI method) (I am a noob at bash, so please forgive me pros)
1.

snap install chromium-ffmpeg && cd /snap/chromium-ffmpeg/current 

2. Find the highest number

cd chromium-ffmpeg-highestnumber && cp libffmpeg.so /usr/lib/x86_64-linux-gnu/opera

Done!

Now I request @xZero or anyone else to fix that script. Once the script is fixed, store the command the run it in a terminal utility known as keep for easy access.

Set this flag to "Wayland".

[opera://flags/#ozone-platform-hint]

No method seems to be working for the version of opera released today (Version 79.0.4143.22, chromium 93.0.4577.63). Replacing the provided libffmpeg.so with any of the provided methods causes the entire page to crash if you try to play any video embedded on it. I have now defaulted back to the provided one.
My go-to method before this was to download the latest version from iteufel's github repo and replace it manually. Had worked since I installed opera.

If for whatever reason your Opera session runs under XWayland (the nested X11 server for Wayland), set the (opera://flags/#ozone-platform-hint) flag to "Wayland."

Set this flag to "Wayland".

[opera://flags/#ozone-platform-hint]

Try to enable wayland in Opera by setting the opera://flags/#ozone-platform-hint
to "Wayland".

The issue seems to have resolved itself on the Cinnamon DE.

I am on Linux Mint 20.3, based on Ubuntu Focal 20.4.3 LTS

I am trying to use the https://tvee.app , but the problem is that opera does not show an option to enable audio sharing for full-screen
and application share.

Even sharing opera tabs' audio doesn't work, though it is the only option which has the check-mark to share audio.
I am fine with letting the main microphone be the monitor of the output, it's just that there needs to be a button to supply sound to the tab.
Here is the output tab (the device TO which I am trying to cast the screen) greying out the mute/unmute button.

@sgunhouse What I do these days to get the selector working is to switch to windowed mode, click on the upload button and launch selector, then switch back to fullscreen. There aren't any problems once you successfully launch the quick selector in any mode, i.e. you can make it pop up in any window mode as long as it has successfully launched once before.

@sgunhouse I think you are right about my Window Manager being the problem, since the selector pops up just fine when I am in windowed mode (as opposed to full-screen). However that begs the question why it suddenly decided to stop working one fine day, as I have been using this feature of Opera for a long time.
The same problem also seems to be occurring in the Opera snap package too.
Changing WMs still doesn't yield a difference though. I have cycled between Mutter, XFWM with Compton and Compositing, Metacity and Compiz.

@sgunhouse Opera 79.0.4143.72 from Opera repositories, Linux Mint 20.2

The Upload quick selector, where you could choose from your recently downloaded files and the clipboard, no longer shows up at all. All that happens now is that the browser becomes unfocused (i.e. one is unable to click anything, but any processes continue as normal) for 40-50 seconds, then returns back to normal without throwing up the quick selector.