Inspector
The inspector allows you to view and edit the contents of an app. To open the inspector, press Ctrl+Shift+I (Cmd+Shift+I on macOS). You can also open the inspector by pressing the Inspect button in the scene context menu.
Inside of the inspector, there is a tree of widgets and a display of the properties of the currently selected widget. You can select an element using the tree or the Select element button, which allows you to select an element in the inspector by clicking on it in the app.
You can edit most widget properties in the inspector, including the styles of a widget once you turn Override style on. The tooltips of the various properties contain useful information, and the Geom is critical for debugging layout issues.
The Debug section of the settings is also useful for debugging any low-level issues.