Pasting from the system clipboard using a Chrome extension.

## background.html
<!DOCTYPE html>
<html>

    <head>
        <script src="background.js"></script>
    </head>

    <body>
        <textarea id="sandbox"></textarea>
    </body>

</html>

## background.js
'use strict';

// A gotcha of sorts with chrome extensions involving clipboard actions is that
// only the content scripts can interact with the page that a user loads. This
// means that we can't put our calls to actually paste into the page in the
// background file, because the background scripts are not able to paste into
// the dom of the page. However, only background pages are able to access the
// system clipboard. Therefore we have to do a little trickery to move between
// the two. We're going to define the functions here to actually read from the
// clipboard into a textarea we've defined in our background html, and then
// we'll get that pasted data from the background page and do the actual
// insertion in our content script. The idea of this comes from:
// http://stackoverflow.com/questions/7144702/the-proper-use-of-execcommandpaste-in-a-chrome-extension
/**
 * Retrieve the current content of the system clipboard.
 */
function getContentFromClipboard() {
    var result = '';
    var sandbox = document.getElementById('sandbox');
    sandbox.value = '';
    sandbox.select();
    if (document.execCommand('paste')) {
        result = sandbox.value;
        console.log('got value from sandbox: ' + result);
    }
    sandbox.value = '';
    return result;
}

/**
 * Send the value that should be pasted to the content script.
 */
function sendPasteToContentScript(toBePasted) {
    // We first need to find the active tab and window and then send the data
    // along. This is based on:
    // https://developer.chrome.com/extensions/messaging
    chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
        chrome.tabs.sendMessage(tabs[0].id, {data: toBePasted});
    });
}

/**
 * The function that will handle our context menu clicks.
 */
function onClickHandler(info, tab) {
    var clipboardContent = getContentFromClipboard();
    console.log('clipboardContent: ' + clipboardContent);
    if (info.menuItemId === 'pasteDemo') {
        console.log('clicked paste demo');
        sendPasteToContentScript(clipboardContent);
    }
}

// Register the click handler for our context menu.
chrome.contextMenus.onClicked.addListener(onClickHandler);

// Set up the single one item "paste"
chrome.runtime.onInstalled.addListener(function(details) {
    chrome.contextMenus.create(
        {
            'title': 'Paste Demo',
            'id': 'pasteDemo',
            'contexts': ['editable']
        });
});

## contentscript.js
'use strict';

/**
 * Insert the text at the cursor into the active element. Note that we're
 * intentionally not appending or simply replacing the value of the editable
 * field, as we want to allow normal pasting conventions. If you paste at the
 * beginning, you shouldn't see all your text be replaced.
 * Taken from:
 * http://stackoverflow.com/questions/7404366/how-do-i-insert-some-text-where-the-cursor-is
 */
function insertTextAtCursor(text) {
    var el = document.activeElement;
    var val = el.value;
    var endIndex;
    var range;
    var doc = el.ownerDocument;
    if (typeof el.selectionStart === 'number' &&
        typeof el.selectionEnd === 'number') {
        endIndex = el.selectionEnd;
        el.value = val.slice(0, endIndex) + text + val.slice(endIndex);
        el.selectionStart = el.selectionEnd = endIndex + text.length;
    } else if (doc.selection !== 'undefined' && doc.selection.createRange) {
        el.focus();
        range = doc.selection.createRange();
        range.collapse(false);
        range.text = text;
        range.select();
    }
}

chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
    if (request.data) {
        insertTextAtCursor(request.data);
    }
});

## manifest.json
{
    "name": "Paste Demo",
    "version": "0.0.1",
    "manifest_version": 2,
    "description": "Demonstration of how to paste in a Chrome extension",
    "permissions": [
        "clipboardRead",
        "contextMenus"
    ],
    "background": {
        "persistent": false,
        "page": "background.html"
    },
    "content_scripts": [
        {
            "matches": [
                "http://*/*",
                "https://*/*"
            ],
            "js": [
                "contentscript.js"
            ],
            "run_at": "document_end",
            "all_frames": false
        }
    ]
}
	<!DOCTYPE html>
	<html>

	<head>
	<script src="background.js"></script>
	</head>

	<body>
	<textarea id="sandbox"></textarea>
	</body>

	</html>
	'use strict';

	// A gotcha of sorts with chrome extensions involving clipboard actions is that
	// only the content scripts can interact with the page that a user loads. This
	// means that we can't put our calls to actually paste into the page in the
	// background file, because the background scripts are not able to paste into
	// the dom of the page. However, only background pages are able to access the
	// system clipboard. Therefore we have to do a little trickery to move between
	// the two. We're going to define the functions here to actually read from the
	// clipboard into a textarea we've defined in our background html, and then
	// we'll get that pasted data from the background page and do the actual
	// insertion in our content script. The idea of this comes from:
	// http://stackoverflow.com/questions/7144702/the-proper-use-of-execcommandpaste-in-a-chrome-extension
	/**
	* Retrieve the current content of the system clipboard.
	*/
	function getContentFromClipboard() {
	var result = '';
	var sandbox = document.getElementById('sandbox');
	sandbox.value = '';
	sandbox.select();
	if (document.execCommand('paste')) {
	result = sandbox.value;
	console.log('got value from sandbox: ' + result);
	}
	sandbox.value = '';
	return result;
	}

	/**
	* Send the value that should be pasted to the content script.
	*/
	function sendPasteToContentScript(toBePasted) {
	// We first need to find the active tab and window and then send the data
	// along. This is based on:
	// https://developer.chrome.com/extensions/messaging
	chrome.tabs.query({active: true, currentWindow: true}, function(tabs) {
	chrome.tabs.sendMessage(tabs[0].id, {data: toBePasted});
	});
	}

	/**
	* The function that will handle our context menu clicks.
	*/
	function onClickHandler(info, tab) {
	var clipboardContent = getContentFromClipboard();
	console.log('clipboardContent: ' + clipboardContent);
	if (info.menuItemId === 'pasteDemo') {
	console.log('clicked paste demo');
	sendPasteToContentScript(clipboardContent);
	}
	}

	// Register the click handler for our context menu.
	chrome.contextMenus.onClicked.addListener(onClickHandler);

	// Set up the single one item "paste"
	chrome.runtime.onInstalled.addListener(function(details) {
	chrome.contextMenus.create(
	{
	'title': 'Paste Demo',
	'id': 'pasteDemo',
	'contexts': ['editable']
	});
	});
	'use strict';

	/**
	* Insert the text at the cursor into the active element. Note that we're
	* intentionally not appending or simply replacing the value of the editable
	* field, as we want to allow normal pasting conventions. If you paste at the
	* beginning, you shouldn't see all your text be replaced.
	* Taken from:
	* http://stackoverflow.com/questions/7404366/how-do-i-insert-some-text-where-the-cursor-is
	*/
	function insertTextAtCursor(text) {
	var el = document.activeElement;
	var val = el.value;
	var endIndex;
	var range;
	var doc = el.ownerDocument;
	if (typeof el.selectionStart === 'number' &&
	typeof el.selectionEnd === 'number') {
	endIndex = el.selectionEnd;
	el.value = val.slice(0, endIndex) + text + val.slice(endIndex);
	el.selectionStart = el.selectionEnd = endIndex + text.length;
	} else if (doc.selection !== 'undefined' && doc.selection.createRange) {
	el.focus();
	range = doc.selection.createRange();
	range.collapse(false);
	range.text = text;
	range.select();
	}
	}

	chrome.runtime.onMessage.addListener(function(request, sender, sendResponse) {
	if (request.data) {
	insertTextAtCursor(request.data);
	}
	});
	{
	"name": "Paste Demo",
	"version": "0.0.1",
	"manifest_version": 2,
	"description": "Demonstration of how to paste in a Chrome extension",
	"permissions": [
	"clipboardRead",
	"contextMenus"
	],
	"background": {
	"persistent": false,
	"page": "background.html"
	},
	"content_scripts": [
	{
	"matches": [
	"http:///",
	"https:///"
	],
	"js": [
	"contentscript.js"
	],
	"run_at": "document_end",
	"all_frames": false
	}
	]
	}