Inspector

An inspector is about the current selection, so it is empty when nothing is selected, and that is exactly what separates it from a settings panel. Settings describe the document or the application and are there whether or not you have clicked anything. An inspector describes the object under the cursor’s last click, changes wholesale when you click a different one, and has nothing to say when you click the background. Every design tool, every code editor with a properties view, and every page builder has one, docked to the trailing edge because that is where a reading eye ends up rather than where it starts.

The contents are properties, grouped, editable in place, and reflecting the object rather than describing it: change a number in the pane and the object moves, move the object and the number follows. Two behaviours are worth designing rather than inheriting. The first is multiple selection, where the pane has to show what the objects share and mark the rest as mixed rather than as blank, which is the same problem a mixed state checkbox solves for one value. The second is what the pane says when nothing is selected: an empty state naming the action that would fill it is worth more than a blank rail, because a reader who sees nothing cannot tell an empty inspector from a broken one.

Its neighbours are easy to confuse because they are all rails. A sidebar is navigation and sits on the leading edge, so it answers where you are rather than what you picked. A drawer is temporary and slides over the content, while an inspector is part of the layout and holds its width. The general family is the supporting pane, one of the canonical layouts, and an inspector is that arrangement with a particular rule about what fills it. When the window narrows, the honest move is usually to turn the inspector into a drawer rather than to squeeze it, since a pane too narrow to hold a labelled number is worse than one summoned on demand.

Two things make or break the feel of it. Reserve the width and the internal geometry, so that switching between a rectangle’s position controls and a text object’s type controls does not resize the pane or shift the canvas beside it. And keep the selection explicit: an inspector that follows hover rather than clicks flickers through every object the pointer crosses, which is why tools that do offer a hover preview keep it visually distinct from the committed selection.