calc- is there a shortcut to expand/collapse groups?

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No predefined keyboard shortcuts, but there are menu items: Data - Group and outline - Show detail/Hide detail.

Those actions can also be assigned to keyboard shortcuts if you want to.

For the show/hide options to work, current selection (cell range) must touch all columns of a horizontal group or all rows of a vertical group. All groups "fully touched" like this will be affected by the show/hide action. If no group is properly selected, the menu items are greyed out.

To select cells in a collapsed range you can use ctrl-A (which will select the entire sheet) or type an address range into the Name field to the left of the formula bar. If you have named ranges, you can also select the names from that field.

You can hide the outline "branch display": Menu Tools - Options, select LibreOffice Calc - View in the left pane and untick Outline symbols.