macOS: First time calling setFixedSize() can cause isMaximized() is true even if it's actually not

On macOS, first time calling setFixedSize() for a widgets-based window can cause isMaximized() return true even if the window is actually not maximized.

See the attachment for minimized reproduceable test program.

Steps to reproduce:

1. Compile the program, execute the binary
2. Try drag the center of the window to move the window

• expected: Can drag the center of the window to move the window
• actual: Dragging the center of the window does nothing

3. Right click the center of the window (will notice the window size is changed)
4. Try drag the center of the window to move the window (now you can move it)

Alternatively:

0. Comment out the `setFixedSize(200, 210);` line in DraggableWidget's ctor, save the edit
1. Compile the program, execute the binary
2. Try drag the center of the window to move the window (you can move it)
3. Right click the center of the window (will notice the window size is changed)
4. Try drag the center of the window to move the window

• expected: Can drag the center of the window to move the window
• actual: Dragging the center of the window does nothing

5. Right click the center of the window again (will notice the window size is changed again)
6. Try drag the center of the window to move the window (now you can move it again)

Additional info:

We do not have this bug on Windows and Linux (tested under KDE x11 and DDE x11)