To reproduce:
1. Create a new project
2. Create a new script
3. Open it with MonoDevelop
4. Create any variable
5. Start typing the name of the variable inside any function
6. When the autocompletion dialog appears and the name of the variable is highlighted, click your mouse anywhere below the current line of text to move the cursor
7. Notice that the autocomplete box hasn't been dismissed
8. Press ENTER
9. Notice that the text between the last symbol you wrote and the current position of the cursor has been deleted.