Map preview not shown if height is relative

Problem/Motivation

When its height is relative (%), a map does not appear on the preview page.
This is a consequence of an openlayers package change: version 5.0.0 does not suffer from it while 6.14.1 does.

Steps to reproduce

• Use openlayers 6.14.1.
• Create a map, set its height to 100%.
• Display preview page.

The browser console says "No map visible because the map container's width or height are 0.".

Proposed resolution

I have tried to fix it with a CSS-only solution, without success for all situations, even with flex containers.
If I set CSS rules to handle relative heights successfully, absolute heights do not display properly anymore.
min-height does not work: the ideal fix would be the openlayers package supports it.
IMO, a non-absolute height should be changed to some default (400px?) in PHP when displaying a map preview, maybe with a warning message.