REGR: Bug in indexing with a CategoricalIndex
closes #16115
closes #16131

# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=h1) Report
> Merging [#16123](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/b8d98618d3dd9a2f912e8ed67b835836406dcb7c?src=pr&el=desc) will **increase** coverage by `0.02%`.
> The diff coverage is `100%`.

[![Impacted file tree graph](https://codecov.io/gh/pandas-dev/pandas/pull/16123/graphs/tree.svg?height=150&src=pr&token=eZ4WkYLtcO&width=650)](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master   #16123      +/-   ##
==========================================
+ Coverage   90.84%   90.86%   +0.02%     
==========================================
  Files         159      159              
  Lines       50779    50796      +17     
==========================================
+ Hits        46129    46156      +27     
+ Misses       4650     4640      -10
```

| Flag | Coverage Δ | |
|---|---|---|
| #multiple | `88.64% <100%> (+0.02%)` | :arrow_up: |
| #single | `40.31% <18.18%> (-0.02%)` | :arrow_down: |

| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/indexes/category.py](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW5kZXhlcy9jYXRlZ29yeS5weQ==) | `98.48% <100%> (+0.04%)` | :arrow_up: |
| [pandas/core/dtypes/common.py](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZHR5cGVzL2NvbW1vbi5weQ==) | `93.24% <0%> (-0.39%)` | :arrow_down: |
| [pandas/core/series.py](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvc2VyaWVzLnB5) | `94.98% <0%> (ø)` | :arrow_up: |
| [pandas/core/generic.py](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZ2VuZXJpYy5weQ==) | `91.31% <0%> (ø)` | :arrow_up: |
| [pandas/plotting/\_converter.py](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=tree#diff-cGFuZGFzL3Bsb3R0aW5nL19jb252ZXJ0ZXIucHk=) | `65.35% <0%> (+1.81%)` | :arrow_up: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/16123?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/16123?src=pr&el=footer). Last update [b8d9861...032232c](https://codecov.io/gh/pandas-dev/pandas/pull/16123?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
 @jorisvandenbossche don't merge yet. not 100% happy with this soln.