Add
in pop-up translation panel

It could have a a standard size of e.g. 3em, and of 99% of the screen width while it has the focus.

Good idea :)

That seems to be doable with JavaScript. Can someone make a prototype I could integrate in the extension?

It can be done with CSS, here is a sample that works with my system/browsers:

<head>
	<style type="text/css">
		.dynwidth {
			width:3em;
		}
		.dynwidth:focus, .dynwidth:hover {
			width:99%;
		}
	</style>
</head>
<body>
	<form>
		|<input class="dynwidth" />|
	</form>
</body>

Purodha Blissenbach‎

Done and live.

Nike‎