Prevent game board from going weird occasionally

under certain circumstances there can be too many tiles in a row.
setting a fixed height in addition to hiding overflow will keep these
from being displayed

index.html

@@ -342,6 +342,8 @@
             ul {
                 float: left;
                 clear: both;
+                height: 1em; /* fallback */
+                height: 10vmin;
                 list-style-type: none;
                 padding: 0;
                 margin: 0;