And just to wrap this topic up:

If an attacker has sufficient control of the computer that they can manipulate the screen capture feature in Vivaldi, then they have the capability to use similar OS-level capabilities. I don't know about Linux, but Windows have the Print Screen key, which can be used take screen captures of the entire workspace, or just an application. And that is much easier to trigger than sending a sequence of (at least 6) mouse events and/or key events to Vivaldi to use its screen capture functionality.

The Vivaldi screen capture functionality is modeled on the Print Screen functionality, but have the following benefits:

It can save directly to file, something which with Print Screen requires pasting it into an app like Paint.

It can be used to select a specific (cropped) portion of the document, which with Print Screen also requires you use a an app like Paint.

It can be used to capture the entire document (which is not possible with Print Screen)

An attacker does not need to have access to your computer to steal your information, they may 1) use phishing or spearphishing to lure you to a site they control, or 2) take over a real site that you visit and trusts, or one of the external sites that they use for tooling, and get your data (including passwords) that way.

If an attacker actually compromises your system they are not going to bother with either a screen capture browser function, or the Print Screen key functionality. They are just going to download and install a bunch of tools on your computer, which may include a keylogger and a full remote control tool known as a RAT (Remote Access Tool) which lets them share and manipulate your display, keyboard, and mouse in real time, and these days they may also (Thanks, Microsoft) install a Recall look-alike without any of the limitations.

(update) BTW, If the user is using the latest browser version, the only way to be compromised (without social engineering and cooperation by the victim) is through the use of an undisclosed zero-day exploit. Those are the purview of two particular groups: Black Hats who don't care about losing access to their exploit once it has been discovered in use, and Nation States that do not deploy any of theirs unless the usefulness of the deployment outweigh the cost of losing access to the exploit because it was discovered and patched, which means that the victim will always be somebody the Nation State "dislikes", which is probably a relatively small population outside those states. Of course, the caching of such exploits by Nation States can backfire, like when the NSA lost control of the EternalBlue exploit.

(update 2) And to be absolutely clear: There is absolutely no security benefit whatsoever to removing the screenshot functionality, since an attacker with that level of access can already do that, and worse, via other means provided by the system. It might look superficially better to you, but it is not actually safer in any way. Removing the screenshot button is similar to putting a piece of cardboard over your house's bathroom window, and claiming that it somehow makes it safer. All it would do is cripple the product for everyone, without giving them any benefit (and if you absolutely insists on getting rid of the screenshot functionality in the toolbar, you can remove the icon from the toolbar, and you can also remove the relevant entries from the menus if you wish). We are happy to make changes that actually improve security, including important changes that might make some things less convenient, but this is not something that needs us to change it.

Unable to save encrypted password to use on another pc