Quote:A future version of Google Chrome will block websites from side-stepping the existing implementation of the Incognito mode to determine whether users are browsing the web in private mode or not.

The Incognito mode in Google Chrome improves user privacy when loading websites, making it harder for advertisers to track them and deliver relevant ads.
And because Incognito has become such a popular way to prevent tracking, many advertisers turned to a simple trick that allowed them to determine whether this private browsing mode was being used when their websites were loaded.

It all came down to detecting whether the FileSystem API is enabled or not, and this is a gimmick that Google has long been aware of. Technically, when browsing the web with Google Chrome, the application creates a FileSystem API to store data. When the Incognito mode is enabled, this feature is automatically disabled to make sure no traces are left behind, as this is the purpose of the private browsing option in the first place.
So what websites did was to look whether the FileSystem API was there or not, basically determining if the Incognito mode was active.