Callstack navigation: automatically expand to next branching node

[violasong]

Navigating the callstack seems slow since it requires two right-arrow key presses to expand and go to next row.

We should change it so that right-arrow (or clicking on disclosure triangle) automatically expands to the next node that has 2 or more children. It should auto-select that node.

Edit: When using left-arrow key or closing the arrow, it should have the opposite behavior - closing all nodes since the last fork.

I think it's possible that expanding all the way to the next branching node will feel like too much. The subtler version would be to just have each right-arrow key press trigger both expand and go to next row.

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