Fix bug in coordsChar

Page-up would sometimes scroll all the way to the top even
though it shouldn't, because of a poorly chosen check in coordsChar.

The new check should no longer get false positives.

lib/codemirror.js

@@ -1707,8 +1707,8 @@ var CodeMirror = (function() {
     }
     // Coords must be lineSpace-local
     function coordsChar(x, y) {
-      if (y < 0) return {line: 0, ch: 0};
       var th = textHeight(), cw = charWidth(), heightPos = displayOffset + Math.floor(y / th);
+      if (heightPos < 0) return {line: 0, ch: 0};
       var lineNo = lineAtHeight(doc, heightPos);
       if (lineNo >= doc.size) return {line: doc.size - 1, ch: getLine(doc.size - 1).text.length};
       var lineObj = getLine(lineNo), text = lineObj.text;