BUG: Incorrect handling of rolling.cov with offset window
 - [x] closes #16058
 - [x] tests added / passed
 - [x] passes ``git diff upstream/master --name-only -- '*.py' | flake8 --diff``
 - [x] whatsnew entry
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=h1) Report
> Merging [#16244](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2aa6e7917000baffb6a14a89d9433bf8409edf5f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16244/graphs/tree.svg?src=pr&token=eZ4WkYLtcO&width=650&height=150)](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16244      +/-   ##
==========================================
+ Coverage   90.34%   90.35%   +<.01%     
==========================================
  Files         164      164              
  Lines       50890    50894       +4     
==========================================
+ Hits        45977    45984       +7     
+ Misses       4913     4910       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.3% <0%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.26% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.58% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.75% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=footer). Last update [2aa6e79...001a88b](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=h1) Report
> Merging [#16244](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/2aa6e7917000baffb6a14a89d9433bf8409edf5f?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16244/graphs/tree.svg?token=eZ4WkYLtcO&src=pr&height=150&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16244      +/-   ##
==========================================
+ Coverage   90.34%   90.35%   +<.01%     
==========================================
  Files         164      164              
  Lines       50890    50894       +4     
==========================================
+ Hits        45977    45984       +7     
+ Misses       4913     4910       -3
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.13% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.3% <0%> (-0.11%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.26% <100%> (+0.01%)` | :arrow_up: |
| [pandas/io/gbq.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2lvL2dicS5weQ==) | `25% <0%> (-58.34%)` | :arrow_down: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.58% <0%> (-0.1%)` | :arrow_down: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `94.75% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.05% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=footer). Last update [2aa6e79...001a88b](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 # [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=h1) Report
> Merging [#16244](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/e60dc4cff2c6e8a2283fbb906faeb8cb01df37ff?src=pr&el=desc) will **increase** coverage by `<.01%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16244/graphs/tree.svg?token=eZ4WkYLtcO&width=650&height=150&src=pr)](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16244      +/-   ##
==========================================
+ Coverage   90.79%    90.8%   +<.01%     
==========================================
  Files         161      161              
  Lines       51063    51067       +4     
==========================================
+ Hits        46365    46369       +4     
  Misses       4698     4698
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <100%> (ø)` | :arrow_up: |
| #single | `40.15% <0%> (-0.01%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/window.py](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvd2luZG93LnB5) | `96.49% <100%> (+0.01%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=footer). Last update [e60dc4c...a5582ec](https://codecov.io/gh/pandas-dev/pandas/pull/16244?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 Rebased. @jreback I think everything was addressed, if you want to verify? thanks!