Don't draw selections outside of the viewport

Issue codemirror#3852

Author: marijnh

lib/codemirror.js

@@ -2312,6 +2312,7 @@
     for (var i = 0; i < doc.sel.ranges.length; i++) {
       if (primary === false && i == doc.sel.primIndex) continue;
       var range = doc.sel.ranges[i];
+      if (range.from().line >= cm.display.viewTo || range.to().line < cm.display.viewFrom) continue;
       var collapsed = range.empty();
       if (collapsed || cm.options.showCursorWhenSelecting)
         drawSelectionCursor(cm, range.head, curFragment);