In the text editor, write "Hello World". Highlight just "Hello" and click on Bold(B icon). "Hello" will be bold and "world" will be normal without the bold. When you click on the text "world" the styling icon bold(B) should get deselect. When you click on text "Hello" , the icon bold(B) should get selected.
In short, it should act like just like this forum editor your a replying.
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