In Chrome 54 we launched a new screen share picker that added tab sharing and separated desktop, windows, and tab options into 3 separate tabs. In Chrome 61 there will be a modification in how the screen share picker works. Today these tabs are always ordered desktop > window > tab, but for privacy or performance reasons a web app might want to promote window or tab sharing as the default choice.
In current Chrome canary and following 61 releases the DesktopCaptureSourceType array ordering will determine which order the window picker will use. If you want to keep the current behaviour, make sure to order your constraints ["screen", "window", "tab"]
Chrome 61 will be rolled out to stable in September. This feature might be turned off with short notice if it doesn't pass the release reviews.