To duplicate this bug:
1. Open Sublime Text 2 Beta Build 2139 (OS: Windows 7 Ultimate).
2. Open two documents.
3. Click View -> Layout -> Columns: 2.
4. Drag one document to the right column.
5. Close the document in the left column. The left column is now empty with a light grey colour.
6. Click view -> Layout -> Single.
7. BUG: The document, which was on the right but is now the only open file on the left, is open but the entire document editing area is light grey and the document can't be seen.
8. To recover the document editing area, I have to click on the document's tab at the top left of the window under the menu bar, which seems to refresh the UI and brings the document back into view again.