Jira Dashboard no longer does lazy-loading of iframe gadgets

Issue Summary

The functionality of lazy-loading of gadgets was introduced in Jira 9.0.
See https://confluence.atlassian.com/jiracore/preparing-for-jira-9-0-1115661092.html#PreparingforJira9.0-inline_gadgetsLazy-loadinginlinegadgets and https://confluence.atlassian.com/jirasoftware/jira-software-9-0-x-release-notes-1142227613.html#JiraSoftware9.0.xreleasenotes-inline_gadgetsLazy-loadinginlinegadgets.
Actually, covering both inline and iframe gadgets since Jira 9.0.

Steps to Reproduce

1. Add several iframe gadgets to a dashboard.
2. Make sure some of those land outside of a viewport on the page load. This can be achieved by tweaking the dashboard layout, zoom-in %, the number of gadgets added to the dashboard.
3. Navigate to the dashboard.
   (observe network requests, especially for the iframes)

This is reproducible on Data Center: yes

Expected Results

Iframes are not eagerly loaded, those belonging to gadgets that landed outside of a viewport on the page load. They should be lazy-loaded, i.e. loaded when a given iframe gadget is about to appear inside the viewport.

Actual Results

All iframes are eagerly loaded (iframe gadgets).

Workaround

Currently there is no known workaround for this behavior. A workaround will be added here when available