Welcome to DT and the forum
Shortcuts affecting screen real estate
Ctrl-Shift-H reveals or hides histogram & Ctrl-Shift-N reveals or hides the Navigation panel
Ctrl-F reveals or hides film strip
Ctrl-H reveals or hides the top header menu tabs
B reveals or hides the borders with arrows
Ctrl-Shift-R reveals or hides the right panel & Ctrl-Shift- L reveals or hides the left panel
Ctrl-Shift- T reveals or hides the top panel & Ctrl-Shift- B reveals or hides the bottom panel
Also note the little triangles and what happens when you click on them.