I did it once, but cannot find out how any more:
how to keep the top row of my LibreOffice Calc sheet visible when scrolling?
(Intuitively it should be either in view or format, at best in right-click menu, but I can't see anything related ... )
You can achieve this by either splitting or freezing the spreadsheet. These two options are slightly different, the main difference being that splitting adds a second scrollbar, viewing the same sheet from two contiguous windows as it were.
How to split the spreadsheet
Using the mouse pointer:
How to freeze the spreadsheet
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