1. The left sidebar mockup is required before this can become an implemetnation task. For now we will do T86135. As part of the definition of done of this task a task for that should be created.

2. The mockup still miss the up/down sorting icons.

Adding the designs to this task for completeness.

I apologize in advance if this is not the right task to comment about it, but as I see the Display menu here I gotta ask, would it be possible to have a check box to see position attributes in world(global) coordinates, please?

The next step for the mockup is how we want the rules to be presented.

Simplified version for the list of filtering rules. No expressions nor conditional operators yet.

Each rule is represented by a collapsable box. The modifiers/panels layout could be used to achieve the collapsing behavior, although dragging/ordering is not necessary (initially), so it's not included in the mockup. An X lets you remove the rule.

Based on the idea by @Máté Kerekes (elfoglalt) of using a similar layout to modifiers but I've removed superfluous functionality.

Perfect, @Jacques Lucke (JacquesLucke) @Hans Goudey (HooglyBoogly) just remember that on top of that if the column is not available we should see it red somehow.

Also the mockup is using position.y and position[y] but we should match the column name with the rules column field. Right now that means position Y`.

As a separate topic (not for this task) we should indeed talk about how to represent vector individual components in the spreadsheet (the other alternative is position[0]). This is assuming we will support this in the nodetree as well,
if not what we have is fine (position X).

To move the other mockups to https://wiki.blender.org/wiki/Source/Editors/Spreadsheet