1. "Right-click taskbar > Properties > Taskbar > Show Windows Store apps on the taskbar" determines whether Store (Metro/Modern) apps show up in the regular taskbar as well as in the "point to upper-left corner for recent apps" area.
  2. Apparently, this has recently acquired a bug where icons for Store apps will remain in the taskbar even after they are tombstoned/killed.
  3. Turning off this setting will clean icons for tombstoned Store apps from the taskbar, but, of course, leaving it off makes it more difficult to work with Store apps that are actually running.
  4. This setting is, for some reason, also responsible for whether the taskbar is accessible by mousing to the bottom of the Start Screen. With it unchecked, there is no way to access the taskbar when running an application that blocks the taskbar, like a video or a game running in fullscreen or windowed borderless modes. Leaving it off is therefore not a viable option.

The part about tombstoned apps not vacating the taskbar is new behavior (to me at least), so I assume this was broken in a recent Windows update (I installed over 200MB of updates earlier today). My new habit to work around this bug will be to uncheck, apply, check, and apply this setting whenever I close Store apps, but is anyone aware of a real fix for this issue?