Middle mouse button tab close bug
If I use my middle mouse button to scroll the page(mouse down, move the mouse up or down, mouse up) and I happen to release the button on any tab, it get closed. This is a bug as this action(closing the tab) should take place only if the mouse down event happened on that tab in the "tab bar" not somewhere on the content of the page.
I agree. I suggest submitting a bug report as this may not have been noticed.
You can avoid the problem by clicking the middle mouse button. After the mouse up event you can scroll up and down without any danger of closing the tab accidentally.