Hi Supriya,
Manually edit files on the Source tab should only be a last resort. If you change something in the page html document - this page is losing touch with the visual editor. That is, you can still add components to create event handlers, but they will be visible only in the visual editor, as IDE can not separate your changes from the code that generates it. IDE stops to make their changes to the page source code in order not to overwrite your changes. The same goes for page JS code and the CSS styles. These files are united by a common name that matches the Screen name.