hmmm... i had tested with 100% height in datagrid. but the 100% depends on the height of the parent container. and so i have looked in the column (parent) and row (parent of column) and a saw these irritating computed heights. So i would show it to you. Because its maybe a bug.
The original reason for my playing/testing was to not use a fixed height in datagrid with virtualization.
But now i used in datagrid 68vh, column parent computed height (default) and row parent computed height (default) and it seems to work fine.
So at the moment it seems that there is no problem at all.
The strange default heights are gone. perhaps i mixed up the height settings.
Sorry for the inconvenience