BUG: Incorrect variable window bounds for first row when offset cover all rows
- [x] closes #42753 
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry

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Allow for cases in which the end bound of the variable window for the first row extends past the last row, that is, cases in which the offset is large enough to cover all the rows.
Hello @dicristina! Thanks for updating this PR. We checked the lines you've touched for [PEP 8](https://www.python.org/dev/peps/pep-0008) issues, and found:



There are currently no PEP 8 issues detected in this Pull Request. Cheers! :beers: 

##### Comment last updated at 2021-08-06 16:42:02 UTC cc @mroeschke  @dicristina can you merge master thanks @dicristina  @meeseeksdev backport 1.3.x Something went wrong ... Please have  a look at my logs.