See bottombar-class-diagram.dia
Each tool is associated to a bottom bar, providing access the tool's options. They're named "optionsbar(s)" in the code, and are managed mostly by the window's "options manager".
Several tools can share the same bottom bar, so the optionsbars are distinct from the tools. Although both work according to a "state" pattern.
While tools are mostly defining the way the image will behave depending on the user input, optionsbars also define actual GTK widgets, which have to be responsive (adapt to mobile phones' window size).
They also have to list options (of course) and actions depending on the current tool, respond to a few keyboard shortcuts (Ctrl+M, Shift+F10, …), and -when it's pertinent- they should provide access to the minimap and/or the zoom controls.
the distinctive feature here is the pair of color picking popovers. The default
associated set of options and actions is pretty rich (color, size, cairo
operator, zoom controls, …) and the tools each provide a Gio.MenuModel to plug
here, with their specific options.
all selection tools have roughly the same actions, which are hardcoded in the optionsbar, although tools can disable actions they don't support (e.g. "Closing selection", useless for the rectangle selection).
each of these tools defines its own optionsbar. All of them have a layout with a "cancel" button at the left and an "apply" button at the right. The rest can differ greatly, but it's usually spinbuttons to provide precise control over numerical values (sizes, angles, percentages, …) in the middle, and a small menu with options at the right.