If you want to implement that, you would need to use a couple of API's.
You need to make sure of Content Scripts in order to capture selection within the DOM. Then you need to use Message Passing to let the Popup communicate to the Content Script. After you do all that, you can simply use chrome.tabs.sendRequest to send a message to the Content Script so that you get back a response with the data.
For example, this is how you can do a Popup that fetches the current selection:
popup.html
<!DOCTYPE html>
<html>
<head>
<style>
body { width: 300px; }
textarea { width: 250px; height: 100px;}
</style>
<script>
function pasteSelection() {
chrome.tabs.getSelected(null, function(tab) {
chrome.tabs.sendRequest(tab.id, {method: "getSelection"}, function (response) {
var text = document.getElementById('text');
text.innerHTML = response.data;
});
});
}
</script>
</head>
<body>
<textarea id="text"> </textarea>
<button onclick="pasteSelection(); ">Paste Selection</button>
</body>
</html>
selection.js
chrome.extension.onRequest.addListener(function(request, sender, sendResponse) {
if (request.method == "getSelection")
sendResponse({data: window.getSelection().toString()});
else
sendResponse({}); // snub them.
});
manifest.json
{
"name": "Selected Text",
"version": "0.1",
"description": "Selected Text",
"browser_action": {
"default_title": "Selected Text",
"default_icon": "online.png",
"default_popup": "popup.html"
},
"permissions": [
"tabs",
"chrome://favicon/",
"http://*/*",
"https://*/*"
],
"content_scripts": [
{
"matches": ["http://*/*"],
"js": ["selection.js"],
"run_at": "document_start",
"all_frames": true
}
]
}