In a recent ToT build (57.0.2926.0), it looks like we're ignoring touch events on the time widget in the (unexpanded) MD system tray a.k.a. status area. I don't know if this has always been broken.

When I tap the time using the touchscreen, nothing happens; in contrast, the system menu is opened if I instead tap the battery, input method, or user photo widgets to its right.

Clicking on the time (and the widgets to its right) *does* open the menu, so I assume that not doing this for touch is just an oversight.