I am new to Qt, and I can't figure out why there is a draggable object in my main window. It starts at the top of the screen in its exapanded form.

And then I can drag it and it shrinks into a square.

You need to remove the main toolbar from either QtDesigner as can be seen below or from the code manually:

Then, you will need to rerun qmake for the changes to take effect as the ui_mainwindow.h
header file needs to be regenerated from the updated mainwindow.ui
description file.

来源:https://stackoverflow.com/questions/27314748/qt-how-to-remove-the-default-main-toolbar-from-the-mainwindow