Screen refresh trigger of an input element value or src change

To which extent does a browser redraw the screen?  Would an element's value change trigger refreshing the screen?  

It turns out to be that the Javascript's setting values of an element, actually does not trigger redrawing. 

document.getElementById("test").value="new value";

What puzzled me was, why then the browser redraw the screen in case with setting an img element's src?

document.getElementById("test").src="data:image/png;base64, ...";

This makes it trickier to implement functions that changes value, or src since the behavior will differ when you would want to retain some values for the session.  

I hit upon this problem when I tried to implement a div section that is shown and hidden at a trigger of pressing a button.  The functionality was implemented in the Javascript function and first I implemented it so that it alter the value of an input element.  It worked.  When you press the button, the Javascript function altered the value of the type='button' input element, without refreshing the screen, that is retaining all the variable values.  Then I switched it so that it would alter an image of an type='image' input element.  Now it does not work.  It reset and initialized the screen, namely to a certain initial state and not the state that was specified as.