Select a row of first non-null values in a sparse table

Using the following table:

A | B    | C    | ts
--+------+------+------------------
1 | null | null | 2016-06-15 10:00
4 | null | null | 2016-06-15 11:00 
4 |    9 | null | 2016-06-15 12:00
5 |    1 |    7 | 2016-06-15 13:00

How do I select the first non-null value of each column in a running window of N rows? "First" as defined by the order of timestamps in columns ts. Querying the above table would result in:

A | B | C
--+---+---
1 | 9 | 7