Viewport cannot be re-docked, tab missing

See the figure below. I floated my viewport and hid the tab, but have no idea how to restore the setting (why the hell i did it in the first place?).

  1. clicking the yellow triangle on the upper left corner has no response
  2. window → reset layout has no effect
  3. edit → editor preference → I can’t find the ''restore" button as other posts mentioned for other versions of ue4.
    Thanks for help.

this problem still exists in the editor for v4.12.5 …