Module body 

CollapsibleBody — a toast body that clamps to a few lines and offers to unfold.

Why a toast body needs a ceiling

A title is one line by construction (ToastSurface builds it .single_line()), but a body is whatever the app hands over — and apps hand over error text. A formatted anyhow chain carries every context frame and every absolute path in it; rendered at toast width that is easily eight or ten lines, and a toast that tall stops being a notification and becomes a dialog nobody agreed to open.

So the body clamps to TOAST_BODY_COLLAPSED_LINES and, only when there is more to see, grows a thin disclosure row. Short bodies — the overwhelming majority — are untouched and gain no chrome.

How “is there more to see” is answered

By measuring, in [Widget::layout_response], at the width the body will actually be given. That is the only place the real content width is known: it is the proposal this widget receives, already net of the glyph, the close button, the padding and the gaps. Deriving it any other way would mean re-deriving the chrome’s arithmetic somewhere else and keeping the two in step forever.

The measured verdict lands in a Signal, which is what drives the disclosure row’s visibility — and writing a signal from layout is the thing to be careful about, since it schedules another pass. Two properties make it safe:

• Transition-guarded. The signal is written only when the verdict changes, tracked in a Cell. A steady state writes nothing, so there is no loop. This is the same discipline BuildContext::activation_signal documents for itself.
• No feedback. The disclosure row sits below the text in a VStack, so showing it changes the body’s height but never its width — and width is the only input to the measurement. The verdict therefore cannot flip as a result of acting on it.

Constants

TOAST_BODY_COLLAPSED_LINES

How many lines of body text a toast shows before offering to unfold.

TOAST_BODY_DISCLOSURE_GAP

Vertical gap between the body text and its disclosure row.

TOAST_DISCLOSURE_ACTION_GAP

Horizontal gap between the disclosure row’s actions.