The reference selection in the Sanger Reads Editor can be out of boundaries

Affected test:

• GUITest_regression_scenarios:test_5739

Scenario:

1. Open any MCA.
2. Scroll to the rightmost position.
3. Click the last base on the reference and drag mouse to the right (out of the widget).
   Expected state: only the last character is selected.
4. Press the Left Arrow key.
   Expected state: the selection moves one base left. Only the base before the last one is selected.
   Current state: the selection moves one base left. Two last bases are selected.