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

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

Start by reproducing the reported drag-and-drop move from [0, 0] to [0, 1] in the task-list element and trace where the destination index is calculated. Done means the same move produces destination [0, 1] rather than [0, 2], with the relevant regression covered by the project's existing tests.

Written by the indexing model from the issue text.