On my side I could reproduce something that resembles to this issue (or maybe is the same root cause) when done the following:
1. Go to Sandbox.WebHome 2. Edit the page in-place 3. Select an entire word from a heading (e.g. Headings) 4. Click 'Source' and observe the selection
When I switched back to 'Source', the view was sometimes on the bottom of the page.
