at the element.
Note that the slider stems from the tiddler - close the tiddler and you close the slider.
So far I've only designed this for the "storyright: how far the left margin of the sidebar is rom the left of the page" - but I hope to make it for all of the "size settings" in the predefined in Theme Tweaks and, in the longer run, also for other settings. In the demo, it is one slider in one tiddler, but it would be useful to have some tiddlers show multiple such sliders via transclusion. - "Everything that has to do with the sidebar measures (height, width, ...)", "Positioning for every main element", etc. The display of these can be from the controlpanel. (This is also in the demo, but it interferes with a previous demo showing sliders centrally placed in the Size settings table... still, opening it activates the above demo.)
CSS can make it so there is no visible slider track, and the button could be designed in any fashion e.g a label tellin which element it concerns and, if no track, an arrow showing directions.
The goal is to make the UI more intuitive.