Stacked folding - content does not fold into the heading

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I am having trouble getting Stacked Folding to work correctly.

Here is a sample doc:
```
## Heading 1
This is the content for my first heading.

## Heading 2
This is the content for my second heading.
```
When I fold it, this is what I get. Notice the content under each heading appears on its own line:
```
## Heading 1
[3 lines]---------------------------
## Heading 2
[3 lines]---------------------------
```
But, this is what I want (and what Masukomi's readme says the stacked folds should look like):
```
## Heading 1 [3 lines]---------------------------
## Heading 2 [3 lines]---------------------------
```

How can I get my stacked folds to look like this?

Reproduce the issue with the sample Markdown headings and compare the current stacked-fold display with the desired README example. Start by reading the repository README and locating the folding behavior for stacked headings; done means each heading and its folded line appear on the same line as shown in the requested output.

Written by the indexing model from the issue text.