Game concept
De Jure Drift
The game also links it as Drifted, Drift, De Jure Drifting.
From the game's encyclopedia
A slow process by which a Title's De Jure Liege can change. It occurs naturally when a Duchy or Kingdom is completely controlled by a Ruler who does not hold the territory's de jure liege title.
This process can be sped up by assigning your Chancellor to the appropriate Councillor Task (Integrate Title). Duchies can only De Jure Drift into Kingdoms, and Kingdoms can only Drift into Empires.
Letting titles De Jure Drift into your Primary Title is a good way to get rid of the Rightful Liege penalty.
Example: The Duchy of Northumbria is de jure part of England, but the territory of the duchy is entirely within Scotland. Since the King of Scotland does not also hold England, the duchy will start to De Jure Drift toward becoming a part of Scotland.
The game's text for patch 1.20.1.2. Words in blue are entries of the encyclopedia: hover one to read it, as in the game.
De Jure Drift is an entry of Crusader Kings III's in-game encyclopedia. The game explains it: "A slow process by which a Title's De Jure Liege can change." It is part of De Jure. The game also links it as Drifted, Drift and De Jure Drifting. Patch 1.20.1.2, from game/common/game_concepts/00_game_concepts.txt and game/localization/english/game_concepts_l_english.yml.
Source: game/common/game_concepts/00_game_concepts.txt, game/localization/english/game_concepts_l_english.yml and the English localisation, Crusader Kings III 1.20.1.2.