Backport PR #2498: Don't catch tab press when something selected

Fixes  #2487

IPython/frontend/html/notebook/static/js/codecell.js

@@ -108,6 +108,7 b' var IPython = (function (IPython) {'
         } else if (event.keyCode === key.TAB && event.type == 'keydown') {
             // Tab completion.
             //Do not trim here because of tooltip
+            if (editor.somethingSelected()){return false}
             var pre_cursor = editor.getRange({line:cur.line,ch:0},cur);
             if (pre_cursor.trim() === "") {
                 // Don't autocomplete if the part of the line before the cursor