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Since updating to Google Chrome 83, you might have noticed that Google Tag Manager's Preview mode no longer works when browsing Chrome in Incognito mode. This is because starting with Chrome 83, third-party cookies are blocked by default in Incognito windows. Google Tag Manager uses third-party cookies to serve browsers in Preview mode with the container draft rather than the live container. There's a simple workaround to make sure Preview mode continues working for any site you want to browse in Preview mode.

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Recently I published an article on how to set up an impact test for the “flicker effect” omnipresent in client-side A/B-testing tools. Be sure to check out that article first to get some context to what we're going to be talking about here. In this short follow-up, I'll show you how to measure the average time of the anti-flicker snippet delaying page visibility, if you choose to deploy the snippet.

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“Flickering” or “Flash Of Original Content” (FOOC) is a phenomenon where there's a (typically) slight but observable delay in the browser updating the site or element layout if the user is included in a variant group for experimentation. This manifests in the original, unmodified element being rendered in the visible portion of the page before the experiment library updates it with the variant. There are ways to mitigate the flicker:

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When custom templates were released for Google Tag Manager, I updated my workflow for working with GTM. Instead of instinctively rushing to the Custom HTML tag and the Custom JavaScript variable, I started considering whether the custom script that needed to be deployed could be transformed into a custom template first. While publishing numerous templates into the community gallery, I always spent some time over the past 12 months tinkering on an extremely complicated template translation: the Snowplow Analytics JavaScript tracker.

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The shadow DOM is a way to add HTML structures to the web page so that they remain isolated from the rest of the document object model (DOM). It's a popular concept with web components, as it allows for encapsulation of web structures so that they aren't affected by style declarations of the parent tree, for example. However, being such a “hidden” structure, anything that happens in the shadow DOM is also hidden from Google Tag Manager's listeners.

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One of the most versatile triggers in Google Tag Manager is the Custom Event trigger. As its name indicates, you can use it to fire your tags when an event is pushed into dataLayer. This process is at the heart of GTM's dataLayer system. And it's not just custom events. Every single trigger type in Google Tag Manager uses the event key in a dataLayer.push(), which is why you'll see events like gtm.

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With the proliferation of gtag.js implementations, we can see that there's a small-ish paradigm shift in how to implement Google's stack of marketing tools. As adding gtag.js snippets to the site code becomes more and more common (to cater to things like early Optimize loading), you might be at a point where you have lots of interesting information stored in the gtag.js queue but no way to access it in your Google Tag Manager tags and variables.

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Simo Ahava

Husband | Father | Analytics developer
simo (at) simoahava.com

Senior Data Advocate at Reaktor

Finland