I have a page with a number of controls among other two TextAreas. I need to
programmatically set certain parts of text bold depending on user interaction
with some of the controls on the page. This however has no effect when I do it.
Using exactly the same code inside a selection change handler for the text area
where I set all the selected text bold works however which makes me think it
has something to do with the text area not having focus when I do it from the
place where its not working from. Setting focus to it first programmatically
however does not solve the problem either. Anyone able to provide some insight ?

How are you doing this? There is a difference in MXML and in AS:

<mx:TextArea fontWeight="bold"/>

myTextArea.setStyle("fontWeight", "bold");

After I posted this it struck me to go in look what the RichText widget does in
flex/frameworks/projects/framework/src/mx/controls/ RichTextEditor.mxml and it
turns out the trick is the following lines:

dispatchEvent(new Event("change"));
invalidateDisplayList();
validateDisplayList();
callLater(setFocus);

immediately after tinkering with the textformat.