We need to see all the CSS and HTML.
For example, in your other thread, you have align-items: center on the body. If you still have that CSS you can try removing it.
Also, if you add height: 100%; to the .flex container it should get the height from the body and pass it down to .msger as well so it can be set using %. Another option is to use vh instead of % for the height on .msger so it doesn’t depend on the parent element for its height calculation.