Navigation in a Rulesheet
Mouse navigation
All general mouse functions are supported:

Click on a cell to select it.

Double-click on a cell to enter edit mode with the cursor at the location you clicked in the line.

Click to open a selected cell's dropdown menu, and then click on the preferred value in the list.

Use the scroll wheel to move up and down the rows in the selected section.
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Keyboard navigation
The Rulesheet window enables navigation through several common keyboard shortcuts in its cell-style sections -- all the sections except Scope:

Conditions and their cells

Actions and their cells

Filters

Rule Statements and their columns
Keyboard shortcuts on a Rulesheet:

Move up: Up-arrow

Move down: Down-arrow, or Enter

Move right: Right-arrow, or Tab

Move left: Left-arrow, or Shift+Tab

Edit content of the cell in focus: Backspace (places the cursor at end of entry)

Clear content of the cell in focus, and then add content: Spacebar

Beginning of line in edit mode: Home

End of line in edit mode: End
Note: Nonconditional actions - In a column 0 value on an Action line, Backspace and Spacebar both toggle the current selection.