The GridSplitter Element includes a set of GridSplitter-only Properties that can be leveraged.
Currently, if you create a DataTrigger on a GridSplitter Element; selecting 'Relative/Self' does not expose to the user these special Properties.

However, targeting the GridSplitter by name instead, exposes all of its available Bindable Properties.

Video attached.

Repro Steps:

1) Create a Studio Project featuring a MainPage.xaml

2) introduce a GridSplitter Element to the Root Grid, and select it to observe its special 'GridSplitter' Properties (such as Resize Behavior, Resize Direction, etc...)

3) Rename the GridSplitter to give it a custom name.

4) With the GridSplitter selected, open the Interactivity/Triggers Panel, and create a new DataTrigger on the GridSplitter.

5) In the Trigger's 'Binding' field, click its 3-Dot menu, and select 'Create Binding'.

6) In the Binding Selector menu, select the 'Relative' Tab, select 'Self' as the 'Relative Mode', then click on the button to the right of the 'Path' field to expose its available Properties to bind to.

Observe how none of the GridSplitter's Special Properties are present in this menu of options.

7) Instead of the 'Relative' Tab, select the 'Name' tab, and in the 'Element Name' field, select the name of the GridSplitter given in Step 3).

8) click on the button to the right of the 'Path' field to expose its available Properties to bind to.

Observe how when targeted by name, all available GridSplitter Special Properties are available to select from.

Expected Results: Targetting a GridSplitter by 'Self' provides the same Binding options as targeting it by 'Name'.