Hello @Anonymous ,
Welcome to Q&A Forum!
According to my research, this is the default design of the SharePoint page section layout.
As a workaround, we suggest that you can click the "Hide header and navigation" button in the upper right corner of the page to display this page in full screen.
Please follow the steps:
1.Edit section
2.On the Section windows, enable "Make this section collapsible" and set "Default display" to Collapsed
3.Publish/Republish page
4.Click "Hide header and navigation" button in the upper right corner of the page
Thanks,
Echo Du
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