Keyboard shortcuts for jumping around

[lihebi]

Old:

• <Esc>: jump to parent
• <Enter>: jump to top-left child

Problem: <Esc> needs to be reserved for close pop-up menus, e.g., right-click menus.

Proposed:

• <shift>-<up>: jump to parent
• <shift>-<down>: jump to top-left child
• <shift>-<left>: follow indegree edge
• <shift>-<right>: follow outdegree edge