Fill Value of textbox on the basis of other textbox Infopath

i am new to infopath. i want to set the value of textbox b based on the value of other textbox a from SharePoint list. In Infopath vsta, textbox a onchanged event i am setting the value of textbox b which work ok as far as i check through preview on infopath. But when i deploy this simple form through administration approve template as content type to my sharepoint document library. the form opens in the web browser but on change event of textbox a is not working (asynchronously). instead it works when i hit the submit button. submission logic is to submit this form to document library. in infopath configuration i have given the form full trust level.

Any Idea why its not working as expect开发者_如何学JAVAed. or is this any other solution apart from infopath programming?

Text box properties -> browser forms -> "Always".

Presence of the OnChanged event handler does not trigger postback (which is needed as code runs on server, not in the browser).