UserAgent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_13_2) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/63.0.3239.132 Safari/537.36

Steps to reproduce the problem:
Open (the work-in-progress demo)
https://tobireif.com/demos/grid/ , 
set the viewport width to approximately 900px.
Scroll down. Right-click -> Inspect -> in the dev tools select the div with class "page", leave the pointer over that element. Notice that below the text "No HTML code needs to be changed" there's too much space inside that Grid item (element "main"), and that it's not caused eg by "margin" or "padding" values. The superfluous space seems to be Grid-related. (Perhaps it's the same size as the Grid gap?)

A stable copy of the page (in case the layout at the above URL changed and doesn't show the issue anymore) is at
https://tobireif.com/non_site_stuff/unfinished_grid_demo_copy_for_grid_span_bug_reports/ .

What is the expected behavior?
There should be no superfluous space at the bottom of the spanning Grid item "main" / its height should be correct.

What went wrong?
There is superfluous space at the bottom of the spanning Grid item "main" / its height is  incorrect.

Did this work before? No 

Does this work in other browsers? No
 I'll link the other reports below.

Chrome version: 63.0.3239.132  Channel: stable
OS Version: OS X 10.13.2
Flash Version: