Starting in version 1.29.8, you can use Clipping to partially clip the visibility of Widgets.
When clipping is enabled on a Widget, only the region of the Widget within the clipping rectangle will be visible. Anything outside of the clipping rectangle will be clipped (hidden).
You can define the clipping rectangle using coordinates (Left, Top, Width, Height measured with respect to the overlay window), or by selecting another Widget to act as the 'Clipping Widget'. When you select a clipping widget, the boundaries of the clipping widget define the visible region of your widget.
Clipping in Theme Editor
Please note: clipping will only work visually when running the theme in ATVO. The Theme Editor overlay editor will not show the clipping in action.
Example setup
In this example I intend to clip Widget1 (the image) to be only visible when it is inside Widget2 (the red rectangle). Widget1 is moving back and forth with an animation.
In the properties of Widget1, find the Display - Clip group. Then enable clipping and select Widget2 as the Clipping Widget. Optionally, you can manually define the rectangle to be clipped via the Left, Top, Width, Height coordinates.
The widgets in the editor:
View image on i.imgur.com
The properties of Widget1 show how clipping is enabled:
View image on i.imgur.com
The result is that only the area of Widget1 within Widget2 is visible:
View image on i.imgur.com
Note that Widget2 does not need to have any visible elements, I just use the border in this case to make it clear where Widget2 is positioned. You can totally hide Widget2, it merely defines the boundaries where Widget1 will be clipped.