Currently, if you are in the process of inputting a value into a Grid's Row or Column Definition Adorner field, and you press [Tab], Studio will switch to any other tab currently open within the application.
This is contrary to expectations, where pressing [Tab] in one Grid Definition Adorner would switch to the next Row or Column input field in the array of definitions.

Repro Steps:

1) Create a new Project containing a MainPage.xaml
2) Create a new Page called MySecondPage.xaml, ensure it is open, and leave it open.
2) Without closing MySecondPage.xaml's tab, open MainPage.xaml, and select the Root Grid within the Page.
3) With the Root Grid selected, hover over its top edge to surface the 'Add Column' Cursor, and click 2-3 times across the top edge to create a few Columns.
4) With the Root Grid still selected and displaying the values of each Column, click on any of its Grid Column Input Fields to edit its values.
This will activate the input field from where a new value can be typed in.
5) Whilst still in this Active Field state, press [Tab].

Observe how the open Page Tab in Studio has changed from MainPage.xaml to MySecondPage.xaml.

Expected Results: Pressing [Tab] on a Grid's Definition field tabs to the next Row or Column in the array.
Expected Results: Pressing [Tab] from an active Input Field simultaneously tabs to the next, as well as serving to confirm the value that was entered when [Tab] was pressed.
Expected Results (To Be Debated): Tabbing from a Grid Definition only tabs between the values of the currently-selected Axis. ie: it is not possible to [Tab] between a RowDefinition and a ColumnDefinition.
Expected Results (To Be Debated): Tabbing from the last Definition of the Grid Definition Array, tabs back to the first Definition in the Axis' array. (ie Tabbing "loops" until [Enter] or [Esc] or a MousePress is made)