replace window.onload with window.addEventListener

[robin-drexler]

PR Summary:

window.onload only allows a single callback to be attached.
As soon as a second callback is added, the previous one is removed.

Some third party scripts contain window.onload handlers which breaks the address forms on customer account pages. The edit form can't be toggled for example.

What approach did you take?

Replaces window.onload with window.addEventListener("load") which supports multiple event listeners.

Demo onload vs addEventListener: https://jsfiddle.net/0ydwoen7/2/
onload 1 is never logged because it's been overridden by the second callback.
addEventListener 2 is shown, however.

addEventListener isn't supported in IE<=8, but I think those are outside of the supported browser matrix and it's already been widely used throughout the code base https://github.com/search?q=repo%3AShopify%2Fdawn%20addEventListener&type=code

Visual impact on existing themes

none

Testing steps/scenarios

• Step 1

Demo links

• Store
• Editor

Checklist

• Added PR summary for release notes
• Requested review from UX (Only for changes that are affecting the experience or perceivable visual details)
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles
• Linted with Theme Check
• Tested on mobile
• Tested on multiple browsers
• Tested for accessibility

[ludoboludo]

Thanks for noticing and creating the fix for it 👍