Mouse does not release in design window in split mode
Create a new HTML document and view it in split mode with Design view on the right. Type some text, then use the mouse to highlight the text, but before releasing drag the mouse to the left into the code view. Then release. If you move the mouse back into the design view, it still thinks the mouse is pressed.
I teach software development and teach students not do make the mistake that leads to this bug. Always check the button status when processing move messages. Don't ever rely on the mouse up message, since it may not occur if the mouse is moved out of the window like this.