column drag-n-drop wrong destination with index

[scottdraves]

to reproduce:

1. in doc/python/TableAPI.ipynb run this:

df = pd.read_csv('../resources/data/interest-rates.csv')
df.set_index(['m3'])

2. then click and start a drag a column like "y1". if you only move the mouse a few pixels for the drag to start, a blue line appears to the right of the "y1" column, indicating the target of the drop, which should not change the table because we are dropping it at its current location.

3. release the mouse to drop the column.

result: the y1 column moves to the left of "y30".
expected result: column stays in its original location.