In the vertical way auto-fill works very easy with a simple double-click on the handler (which is located in the lower right edge of the marked cell). Then the whole column is getting auto-filled until the neighbour column on its left hand side is not occupied anymore.
How can I do the similar thing like auto-fill a horizontal row until the neighbour row above is not occupied anymore?
According to https://help.libreoffice.org/Calc/Automatically_Calculating_Series:
You can double-click the fill handle to automatically fill all empty columns of the current data block.
However, there is no similar feature for rows. Instead, to fill many cells, do the following:
Alt+s, i, r on my machine).Tools → Customize can be used to set up a key combination to fill right.
For only a few cells, simply drag and drop the fill handle instead.
Thank you very much! Maybe it would be useful if the LibreOffice-developers could add such a feature for columns also for rows. For example: Press Shift and double click the handler to auto-fill a row instead of a column.