There is a web platform test for this (viewport-resize-event-on-iframe-size-change.html) it got merged into our tree and automatically marked failing in bug 1713003 on June 8. Visual viewport api got enabled in bug 1551302 on June 1. No one who knew about visual viewport api and it getting enabled on desktop knew about the test I don't think. Unfortunate.
Bug 1725569 Comment 2 Edit History
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There is a web platform test for this (viewport-resize-event-on-iframe-size-change.html) it got merged into our tree and automatically marked failing in bug 1713003 on June 8. Visual viewport api got enabled in bug 1551302 on June 1. No one who knew about visual viewport api and it getting enabled on desktop knew about the test getting added and marked as failing I don't think. Unfortunate.