Created attachment 204803 [details]
reproducer file

1. Open the attached reproducer file
2. Go to the Heading 3 , Select it and change the text style to Heading 3
3. Notice that in Heading 3, there is a textbox-like wrap around it

See also the video attached to this report

Reproducible: Always

User Profile Reset: No

A bibisect traces this issue to commit: 
 7a35f3dc7419d833b8f47069c4df63e900ccb880

https://gerrit.libreoffice.org/c/core/+/175580

```
tdf#48459 sw inline heading: apply it on the selected words

Selected text at the beginning of a paragraph (<= 75 characters)
become text frame based inline heading at applying a paragraph style
(using Formatting toolbar, context menu, Ctrl-1...Ctrl-5 or
Styles sidebar panel).

If the whole paragraph is selected, or if no or multiple
paragraphs are selected, formatting is still applied on the
whole paragraphs.

Using text frames for inline heading is ODF 1.0 compliant
and fully back-compatible with the older Writer versions.

The new inline heading frame contains direct formatting
to zero the upper and bottom paragraph margin to solve
interoperability issues: in MSO, margins of heading styles
are zeroed by using the style separators.

Note: lack of inline heading was a showstopper for creating
APA-, IEEE-, MIL-STD-961E-format, legal etc. documents.

Note: recent Formula frame style will be replaced by the
planned Inline Heading, which will be used by the DOCX filter
to export OOXML style separators instead of text frames.

Change-Id: I6722dcaef046bdbca2fe044d175806fa8c65278c
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/175580
Tested-by: Jenkins
Reviewed-by: László Németh <nemeth@numbertext.org>
```