TreeHerder has a bottom panel with this styling: flex: 0 0 auto; height: 35%; value But we don't actually need that trial layout -- it will never actually influence our sizing, because: (0) The 'height' is definite (there's a valid container-height for 35% to resolve against, in this case). So it's effectively a fixed value, let's say 70px. (1) The flex item has "flex-shrink:0", so it's not in danger of shrinking *below* its flex base size (of 35% == e.g. 70px) (2) "min-height:auto" takes definite "height" values into consideration, and will lower its value as-necessary to accomodate them. So: if the content height happens to be *smaller* than 70px, then we're in no danger of violating it (since flex-shrink is 0 and our flex base size is larger than our min-height), so it doesn't actually impact layout. And if the content height happens to be *larger* than 70px in this example, then we'd end up picking "70px" as the final size for min-height:auto, so the content height won't actually matter. So, there's no actual need to measure the content height.