StreamTopSearch PUP
StreamTopSearch is the name of a browser extension that exhibits functionality similar to that of browser hijackers and potentially unwanted programs.
Potentially unwanted programs or PUPs are neither legitimate software nor as dangerous as full-on malware. PUPs occupy a gray space between those two categories and will exhibit some features that, while not outright dangerous, will still diminish the user's experience, hence their name.
StreamTopSearch, once installed inside your favorite browser, will modify a number of settings without asking for explicit user consent. These include changing the browser's default search engine to the StreamTopSearch one and modifying the browser's home page and the contents of the new tab window.
The fact that those changes are performed without explicit user consent and with poor disclosure of what is happening is what puts StreamTopSearch into the category of potentially unwanted programs.
Similar PUPs can get on your system through several avenues, the most common of which is bundle install packages. Bundle installers will often contain one legitimate program, with one or more potentially unwanted programs piggy-backing inside the same install package.
The best way to steer clear of similar software is to always source your downloads from secure and properly curated download portals.