Github recently has added a setting to enable fixed width (monospace) font for their text/markdown editor. This can be found at Settings -> Appearance -> Markdown editor font preference.

I have enabled that option and IMHO it makes it a lot easier to write correctly indented markdown but more importantly to write correctly indented code blocks in issue reports.

It is easier to reason about the final rendered result if you can see the entered text as

```
test
|-- test
    |-- test (4 spaces)
```

instead of seeing the text as

``` test |-- test     |-- test (4 spaces) ```

In both cases the final rendered result is

test
|-- test
    |-- test (4 spaces)