Fix duplicated content and indentation in hide-markdown-diff

[cheap-glitch]

Fixes #4035

Test URLs

Example: https://github.com/sindresorhus/refined-github/edit/main/readme.md

For a more thorough testing:

Create a Markdown file with the following content:

paragraph

paragraph

paragraph [REMOVED]

# level 1 heading
# level 1 heading
# level 1 heading [REMOVED]

## level 2 heading
## level 2 heading
## level 2 heading [REMOVED]

### level 3 heading
### level 3 heading
### level 3 heading [REMOVED]

* list item
* list item
* list item [REMOVED]

1. list item
2. list item
3. list item [REMOVED]

> quote
>
> quote
>
> quote [REMOVED]

[link](https://www.example.com)
[link](https://www.example.com)
[link](https://www.example.com)
[link [REMOVED]](https://www.example.com)

`inline code`
`inline code`
`inline code [REMOVED]`

```
code block
```
```
code block
```
```
code block [REMOVED]
```

```javascript
// code block with language
```
```javascript
// code block with language
```
```javascript
// code block with language
```
```javascript
// code block with language [REMOVED]
```

![image](https://fakeimg.pl/400x100/?text=image)
![image](https://fakeimg.pl/400x100/?text=image)
![image](https://fakeimg.pl/400x100/?text=image)
![image](https://fakeimg.pl/400x100/?text=image%20[REMOVED])

paragraph → heading

#### heading → paragraph

* list item → paragraph

paragraph → list item

> quote → paragraph

paragraph → quote

> quote → heading

#### heading → quote

> multi-line quote → paragraph
>
> multi-line quote → paragraph

> multi-line quote → heading
>
> multi-line quote → paragraph

Then edit it and replace its content with:

paragraph

paragraph [CHANGED]



# level 1 heading
# level 1 heading [CHANGED]


## level 2 heading
## level 2 heading [CHANGED]


### level 3 heading
### level 3 heading [CHANGED]


* list item
* list item [CHANGED]


1. list item
2. list item [CHANGED]


> quote
>
> quote [CHANGED]



[link](https://www.example.com)
[link [CHANGED]](https://www.example.com)
[link](https://www.example.com/CHANGED)


`inline code`
`inline code [CHANGED]`


```
code block
```
```
code block [CHANGED]
```




```javascript
// code block with language
```
```javascript
// code block with language [CHANGED]
```
```typescript
// code block with language
```




![image](https://fakeimg.pl/400x100/?text=image)
![image [CHANGED]](https://fakeimg.pl/400x100/?text=image)
![image](https://fakeimg.pl/400x100/?text=image%20[CHANGED])


#### paragraph → heading

heading → paragraph

list item → paragraph

* paragraph → list item

quote → paragraph

> paragraph → quote

#### quote → heading

> heading → quote

multi-line quote → paragraph
multi-line quote → paragraph


## multi-line quote → heading
multi-line quote → paragraph

This PR:

• removes the decorations on added/changed inline code
• fixes wrong text color applied to changed blockquotes
• fixes the GitHub bug where no indentation is applied to the contents of changed quote blocks
• fixes the GitHub bug where a multi-line quote block changed into a paragraph may contain some duplicated content (see Fix duplicated content and indentation in hide-markdown-diff #4052 (comment) for a minimal reproducible example)

[fregante]

I'm having a hard time seeing the problem with your demo markdown. This is what I see without this PR

The only issue is the green text, but I see no duplicate content, I think

[cheap-glitch]

It's around the end:

With diff	Without diff

The first part of the quote block gets duplicated because it's not marked as deleted.

---

Here's a minimal reproducible example:

Base	Modified
> one [DUPLICATED] > > two > > three	one [DUPLICATED] two three

This will give the following diff:

With diff	Without diff (broken)

[cheap-glitch]

In the end I had to rely on some JS to fix the missing indentation issue.

At first I applied a margin on any siblings of .changed_tag[data-before-tag="blockquote"] but this causes issues with inline <ins> elements, so it has to be applied to the diff block wrapper.

[fregante]

Needs a little more work, I think.

Right side: this PR

• the duplicate line is still duplicate in this view, it just changed style

• also the code block changed style in the final result

• The PR title needs to be more specific than "Fix X" unless the feature is completely broken
• The MutationObserver line needs a comment explaining why it's there or what it's waiting for

[cheap-glitch]

the duplicate line is still duplicate in this view, it just changed style

Actually since I switched to the CSS-only solution the style doesn't change anymore. Anyway, I thought it made more sense to hide the duplicated content only when hiding the whole diff — otherwise we're modifying the native GH behavior even when the feature isn't toggled on, which isn't really what's advertised.

[fregante]

• the duplicate line is still duplicate in this view, it just changed style

By this I meant that on the right, the line has a red background. Either we drop it or leave it unchanged since there's already a red-text line below it

[cheap-glitch]

Since d069190 (#4052) the duplicated content isn't marked in the diff anymore. The only change is the missing indentation that's being re-added:

Vanilla	RGH

[fregante]

It seems to work correctly