When working with AutoFilter, moving between two sheets that have an
AutoFiltered range, sometimes the AutoFilter dropdowns disappear.

To reproduce:
1) Create a new spreadsheet
2) Insert the following sample data into Sheet1 and Sheet2
a	b	c	d	e
1	2	3	4	5
4	5	6	7	8
3	2	3	4	6

3) On Sheet1, select the entire range above and click Data->Filter->AutoFilter.
Dropdowns appear on the 1st row (a, b, c, d, e).

4) On Sheet 2, select the entire range above and click Data->Filter->AutoFilter.
Dropdowns appear on the 1st row (a, b, c, d, e).

5) Return to Sheet1 (by clicking "Sheet1" tab) and notice the dropdowns
disappeared on Sheet1.

6) Workaround: Re-select the range and repeat step 3.