BUG: rolling_* functions should not shrink window
Closes https://github.com/pydata/pandas/issues/7764.

can you add in tests for the rest of the rolling_\* function? (I think make them work rather than raising, by returning all NaN). Not sure when that was changed (ideally if you can figure THAT out would be gr8, we could then reference that issue)
 Good point. rolling_min() and rolling_max() also have this problem. Am fixing those two, too..
 It looks like this behavior was introduced as follows:
`rolling_cov`: 11/30/2012, so looks like from v0.10.0;
`rolling_corr`: 3/28/2014, so looks like from v0.14.0;
`rolling_min/max`: < 11/30/2012 (`window = min(window, n0)` is in the entire history of https://github.com/pydata/pandas/commits/master/pandas/algos.pyx), so from <= v0.10.0
Though short of actually installing and testing old versions, I can't be 100% certain...
 looks pretty good!

pls add a note in v0.15.0.txt - put under API changes, as it technically now will work correctly. I would do a small example their to illustrate the 'new' behavior.
 I added a note to v0.15.0.txt, but afraid don't know how to "test" it. Let me know if I did something wrong...
 You create a block something like this (obviously NOT the content). and its like an ipython session.

```
.. ipython:: python

   index = date_range('1/1/2000', periods=8)
   s = Series(randn(5), index=['a', 'b', 'c', 'd', 'e'])
   df = DataFrame(randn(8, 3), index=index,
                  columns=['A', 'B', 'C'])
   wp = Panel(randn(2, 5, 4), items=['Item1', 'Item2'],
              major_axis=date_range('1/1/2000', periods=5),
              minor_axis=['A', 'B', 'C', 'D'])
```
 OK. Will try to give a more explicit example tomorrow or over the weekend.

BTW, is there an easy way for me to preview what the results will look like?
 Thank you very much guys! Appreciate all the work.
 @seth-p you can build the docs see here: http://pandas.pydata.org/developers.html#contributing-to-the-documentation (it will be build in the dev docs when its merged as well).
 @seth-p can you update the docs as discussed above...thxs
 @jreback, I tried to follow http://pandas.pydata.org/developers.html#contributing-to-the-documentation, but am stumped by the error below. Any idea what that's about? Is there anything else I need to install besides sphinx? Apologies if I'm missing something obvious...

```
C:\Users\seth\Documents\github\pandas\doc>python make.py html
Running Sphinx v1.2.2
loading pickled environment... failed: [Errno 2] No such file or directory: 'C:\\Users\\seth\\Documents\\github\\pandas\\doc\\build\\doctrees\\environment.pickle'
[autosummary] generating autosummary for: 10min.rst, api.rst, basics.rst, categorical.rst, comparison_with_r.rst, comparison_with_sql.rst, computation.rst, contributing.rst, cookbook.rst, dsintro.rst, ..., r_interface.rst, release.rst, remote_data.rst, reshaping.rst, rplot.rst, sparse.rst, timeseries.rst, tutorials.rst, visualization.rst, whatsnew.rst
Exception occurred while building, starting debugger:
Traceback (most recent call last):
  File "C:\Python34\lib\site-packages\sphinx\cmdline.py", line 253, in main
    warningiserror, tags, verbosity, parallel)
  File "C:\Python34\lib\site-packages\sphinx\application.py", line 139, in __init__
    self._init_builder(buildername)
  File "C:\Python34\lib\site-packages\sphinx\application.py", line 201, in _init_builder
    self.emit('builder-inited')
  File "C:\Python34\lib\site-packages\sphinx\application.py", line 400, in emit
    results.append(callback(self, *args))
  File "C:\Python34\lib\site-packages\sphinx\ext\autosummary\__init__.py", line 526, in process_generate_options
    base_path=app.srcdir)
  File "C:\Python34\lib\site-packages\sphinx\ext\autosummary\generate.py", line 110, in generate_autosummary_docs
    items = find_autosummary_in_files(sources)
  File "C:\Python34\lib\site-packages\sphinx\ext\autosummary\generate.py", line 227, in find_autosummary_in_files
    lines = f.read().splitlines()
  File "C:\Python34\lib\encodings\cp1252.py", line 23, in decode
    return codecs.charmap_decode(input,self.errors,decoding_table)[0]
UnicodeDecodeError: 'charmap' codec can't decode byte 0x9d in position 794: character maps to <undefined>
> c:\python34\lib\encodings\cp1252.py(23)decode()
-> return codecs.charmap_decode(input,self.errors,decoding_table)[0]
(Pdb)
```
 use sphinx 1.1.3 (something is broken in more recent version)
 Will give it a shot, thanks.

Note that README.rst say: "Building the docs with Sphinx version 1.2 is broken. Use the latest stable version (1.2.1) or the older 1.1.3.". This suggests v1.2.2 should work...
 I get the same error with Sphinx v1.1.3. :-(
 maybe look thru some issues about building on docs on windows 
I think it is non trivial
@jorisvandenbossche did a lot of work in this - he's away for a bit I believe
 Looks like I have the exact same error @phaebz described on https://github.com/pydata/pandas/pull/5530#issuecomment-33325721. Afraid I can't make heads or tails out of the discussion there...
 oh thought u were building on py 2.7 ? on windows
iirc that was. 3.3 - which I don't think works (problem with sphinx or something)

ok no biggie just edit and I'll take a look at your docs
 I'm using 3.4.1 (64-bit, if that matters) on Windows. OK, will commit changes soon. Sorry for the inconvenience...
 I checked in the updated docs. Travis build is still running... Let me know if you had something else in mind for the docs.
 merged via https://github.com/pydata/pandas/commit/dbb6185975c562abaccafe8dd2e17866d18751ed
 thanks!
 yes, it is a python 3 issue what you got when building the docs. I should update the README to mention that for now it only fully works for python 2 (or better fix it on python 3)
