Created attachment 159013 [details]
screenshot 1 - Adwaita light theme

When rows or columns are hidden in a spreadsheet, and either you forgot that you had hidden rows/columns or you were given a spreadsheet by someone else and you don't pay very close attention to the row numbers or column names, it's very easy to not notice that there are hidden rows/columns at all.

Calc indicates the "hidden" state by simply doubling the 1 pixel gray separator line between the surrounding rows/columns. The problem is that my eyes can't spot that unless I'm paying extreme attention or I'm 10-20 centimeters from the screen.

This is already difficult with a light theme (like GTK's official "Adwaita" theme) as you can see in the 1st attached screenshot here. Try "instantly" noticing the hidden portions without checking numbers or letters sequences; unless you're told that there are hidden contents, you most likely won't ever notice.

Then, if you are using a dark theme (like "Adwaita-Dark" in the 2nd screenshot) it is downright impossible to spot the hidden rows and columns, even if you know they are supposed to be somewhere in there.


The 2px line instead of 1px is not obvious enough, particularly on a laptop screen. Some suggestions I could offer:
- Thicker (4px?) line, and/or:
- More opaque line (black/white instead of gray), or colored (ex: red/orange)

I tried simulating a 4px thick line on my end and it was much easier for discoverability.

Thanks for considering this!