use min-height to support unsupporting browsers

Author: silverwind

web_src/css/features/heatmap.css

@@ -8,7 +8,7 @@
   #user-heatmap {
     /* Set element to fixed height so that it does not resize after load. The calculation is complex
        because the element does not scale with a fixed aspect ratio. */
-    height: calc((100cqw / 5) - (100cqw / 25) + 20px);
+    min-height: calc((100cqw / 5) - (100cqw / 25) + 20px);
   }
 }