Main Window minimal size constraints

The main window in my application embeds several widget within the main window using the QStackedWidget. When the application is run the main window can not be resized below certain size, while there certainly is enough free space to go even narrower.

Every widget (the main window and the embedded widget) can be resized to, lets say, 100x100 in Qt Designer, while the application can not become smaller than 300x300.

Where are the size constraints set? How do I remove them?

You can use:

Code:

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mainWindow.setMinimumSize(100, 100);

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Are you using layouts? Because from what i have understand might be some layout-ing issue.
See if you can reproduce the problem in a small compilable example, and attach it here.