Chrome extension content script re-injection after upgrade or install

The only way to force a content script to be injected without refreshing the page is via programatic injection.

You can get all tabs and inject code into them using the chrome tabs API. For example you can store a manifest version in local storage and every time check if the manifest version is old one (in background page), if so you can get all active tabs and inject your code programmatically, or any other solution that will make you sure that the extension is updated.

Get all tabs using:
chrome.tabs.query

and inject your code into all pages
chrome.tabs.executeScript(tabId, {file: "content_script.js"});

There's a way to allow a content script heavy extension to continue functioning after an upgrade, and to make it work immediately upon installation.

Install/upgrade

The install method is to simply iterate through all tabs in all windows, and inject some scripts programmatically into tabs with matching URLs.

Obviously, you have to do it in a background page or event page script declared in manifest.json:

"background": {
    "scripts": ["background.js"]
},

background.js:

// Add a `manifest` property to the `chrome` object.
chrome.manifest = chrome.runtime.getManifest();

var injectIntoTab = function (tab) {
    // You could iterate through the content scripts here
    var scripts = chrome.manifest.content_scripts[0].js;
    var i = 0, s = scripts.length;
    for( ; i < s; i++ ) {
        chrome.tabs.executeScript(tab.id, {
            file: scripts[i]
        });
    }
}

// Get all windows
chrome.windows.getAll({
    populate: true
}, function (windows) {
    var i = 0, w = windows.length, currentWindow;
    for( ; i < w; i++ ) {
        currentWindow = windows[i];
        var j = 0, t = currentWindow.tabs.length, currentTab;
        for( ; j < t; j++ ) {
            currentTab = currentWindow.tabs[j];
            // Skip chrome:// and https:// pages
            if( ! currentTab.url.match(/(chrome|https):\/\//gi) ) {
                injectIntoTab(currentTab);
            }
        }
    }
});

ManifestV3

manifest.json:

"background": {"service_worker": "background.js"},
"permissions": ["scripting"],
"host_permissions": ["<all_urls>"],

These host_permissions should be the same as the content script's matches.

background.js:

chrome.runtime.onInstalled.addListener(async () => {
  for (const cs of chrome.runtime.getManifest().content_scripts) {
    for (const tab of await chrome.tabs.query({url: cs.matches})) {
      chrome.scripting.executeScript({
        target: {tabId: tab.id},
        files: cs.js,
      });
    }
  }
});

This is a simplified example that doesn't handle frames. You can use getAllFrames API and match the URLs yourself, see the documentation for matching patterns.

Historical trivia

In ancient Chrome 26 and earlier content scripts could restore connection to the background script. It was fixed http://crbug.com/168263 in 2013. You can see an example of this trick in the earlier revisions of this answer.

Try this in your background script. Many of the old methods have been deprecated now, so I have refactored the code. For my use I'm only installing single content_script file. If need you can iterate over chrome.runtime.getManifest().content_scripts array to get all .js files.

chrome.runtime.onInstalled.addListener(installScript);

function installScript(details){
    // console.log('Installing content script in all tabs.');
    let params = {
        currentWindow: true
    };
    chrome.tabs.query(params, function gotTabs(tabs){
        let contentjsFile = chrome.runtime.getManifest().content_scripts[0].js[0];
        for (let index = 0; index < tabs.length; index++) {
            chrome.tabs.executeScript(tabs[index].id, {
                file: contentjsFile
            },
            result => {
                const lastErr = chrome.runtime.lastError;
                if (lastErr) {
                    console.error('tab: ' + tabs[index].id + ' lastError: ' + JSON.stringify(lastErr));
                }
            })
        }
    });    
}