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
        }
    ]
}