In calc you can select all rows and columns by clicking the blank space at the start of the row and column headers. This space being blank it isn't obvious that this area does anything. It would be nice to indicate visually that this area does something when clicked on. Please consider adding an icon to indicate that an action is available.
Upper left corner of calc sheet has no visual indication that it has an action attached to it.
Add a visual indication that this area of a calc spreadsheet have an action that can be selected
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While not a priority this would be a nice visual touch for new users.
This is the version provided by OpenSUSE 15.5
I agree this would improve discoverability.
Both Office 365 and OnlyOffice have a triangle corner pointing at the sheet.
A "Select all cells" tooltip could also help.
I wonder if an icon would be more flexible. But no doubt for the tooltip.
Code pointer: class ScCornerButton in sc/source/ui/view/tabview.cxx