In the documentation about the ruler  there is no information about the possibility to change the left paragraph indent by using the ctrl-key and the bottom left triangle.
See also the discussion in bug 119076.
I can certainly confirm that mention of CTRL+drag is missing from this page (of documentation) and that it is a very useful thing to know!
Moreover it says "You can also double-click anywhere on the horizontal ruler, and adjust the indents in the Paragraph dialog." without mentioning that doing so creates a tab stop too!
Personally I find it odd to refer to these drag handles as "triangles" as well, when they are not (even) triangles. I've struggled to find what Microsoft call theirs because their documentation is so lame.
Of course, they way they do it is they split that drag handle into a top and bottom part and if you drag the bottom part it's equivalent to what LibreOffice Writer does, and if you drag the top part it's the equivalent to what LibreOffice Writer does when CRTL is held down. In short they provide a visual cue on screen to explore and aid in discovery.
Created attachment 147848 [details]
My submission for bug 119657 HELP: Add informations about changing left paragraph indent of the ruler with CTRL
My first submission bug fix for Libre Office document ion. Please go easy of the flaming.
Thanks for the addition
* more info on rulers: https://help.libreoffice.org/6.2/en-US/text/shared/02/13020000.html?&DbPAR=WRITER&System=UNIX, possibly covering the bug subject. A 2-way link between the 2 help pages can be enough.