Gwenview is always displayed in full screen and I have no more control to pass it in window mode. This is even more annoying that I do not see the taskbar anymore. Is this a checkbox in the Preferences? A combination of keys?
Please provide clear steps to reproduce the problem (i.e. describe exactly what you did), add what you expected to happen and list what happened instead. In general the F11 key toggles between fullscreen and normal/windowed mode.
Without the requested information, we are unable to proceed. Closing for now.