Moving an item from `sourceIndex = 0`, sets `dst` as `newIndex + 1`

[jlduran]

For example:
Moving Hubot from position [0, 0] to position [0, 1] actually gets calculated as moved to position [0, 2].

[jlduran]

Reading the comment in:

task-lists-element/src/sortable.ts

Line 140 in 78fff62

// Increment insertion index when moving item down in its own list.

It appears that this behavior is explicit.