Prepositions: "in" vs. "on" a tab/widget

Although a tab is technically a specific type of container element which you would think calls for "in" as the preferred preposition, the usage metaphor treats it as a sheet of paper, ONto which things are placed or written. A widget, although also a container element, is a more generic term; thus it is treated as an undifferentiated container, and then the metaphor clearly calls for things to be placed INto it.

The inherent problem here is drawing the line between the real world and the online world. Some people are hesitant to make a distinction that implies that something that is otherwise virtual would be an element of the real world.

However, information is described using in when it occurs inside of an object. On is used to refer to information that is clearly visible on the exterior of an object.

Added on may refer to extending the functionality of the tabs. For instance, a synonym of add-on when referring to a software program would be an extension.

If the widgets are clearly visible, and appear or extend the exterior of the tabs, then they would be added on to the tabs. If the widgets appear in the center of the tabs, you could say that have been added into the tabs, as a sort of inlay.