window.TrelloPowerUp.iframe().alert() in a frame (
card-back-section), but an alert is displayed under the modal underlay.
This is expected behavior when using t.alert(). The iframe is rendered on the board, so an alert’s going to show up on the board level rather than in front of the modal.
I call t.alert() in the card-back-section context which is rendered on the card modal.
While the alert is being called in the card-back-section, we render alerts from a fixed position on the board, so it will show up underneath the modal. I suggest you use t.popup() for rendering on the card-back-section.