Revealing selection is broken for editors < 15 lines high

[tsmaeder]

To reproduce:

1. Open a Java class that is more than 15 lines long
2. Add "System.out.println()" some place after the 15th line
3. Do a "Find Usages" (Alt-F7)
4. Resize the editor to 10 lines high
5. In the usages view, double click the occurrence you have created in step Package docker runner in Che #2
6. Observe: there is no selection visible in the editor.

The same thing works when the editor is > 15 lines high.

[tsmaeder]

The problem is in OrionDocument.java:

  public void setSelectedRange(LinearRange range, boolean show) {
    int startOffset = range.getStartOffset();
    editorOverlay.setSelection(startOffset, startOffset + range.getLength(), show);

    TextPosition position = getPositionFromIndex(startOffset);
    if (show && position != null) {
      int lineNumber = position.getLine();
      int topIndex = lineNumber - MARGIN_TOP;
      editorOverlay.getTextView().setTopIndex(topIndex > 0 ? topIndex : 0);
    }
  }

This will always set the top index to 15 less than the selection line, which is bogus if the editor is less than 15 lines high.
I would suggest two improvements:

1. Don't scroll if the selection is already visible. This makes the editor position jump less.
2. if less than 30 lines are available, we should put the selection in the middle of the editor. In this way, we maximize the context we show before and after the line in question.

One other thing I noticed is that on one hand, we pass the "show" flag on to the editor overlay, on the other hand, we're setting the top position by hand. If we want to control the editor position, shouldn't we alsways pass "false" the the editor overlay?