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It is possible to create a scroll bar with markdown.
Do you use any specific code?

I did not use code. So if I dont format the text with markdown then this is what happens?

No. The other way. If you use code then this could happen.
I prepare my text in an editor and copy it to steemit.
Do you also use an editor/tool and then copy/paste?

Yes I use LibreOffice for writing and spell checking. I use bluefish for html and php. So you write the markdown when you are making the post on steemit correct?

I had a similar problem in the past. I created my text in OneNote (with the markdown code) and copied it to steem. This caused a lot of problems with strange behaviours.
Now I'm using a simple texteditor (notepad++) and everything is fine.