'Close tabs' button in BB-code editor deletes part of the tagged text

Steps to reproduce:
1. Press 'New Topic' button - BB-editor will opens.
2. Add seguented text, marked with different 'Margin size' tags (see screenshot 'screen_before.jpg')

[indent=20]I never [/indent]
[indent=20] make stupid [/indent]
[indent=25] mistakes[/indent]

3. Press 'Close tags' button

AR: Now text looks like this:
[indent=20]
[indent=20] make stupid [/indent]
[indent=25] mistakes[/indent][/indent]

• part of the text is deleted*, extra tags added (see screenshot 'screen_after.jpg')

ER: text shouldn't be changed.