AdaptiveMasterDetail class

List and detail side by side when there is room, list alone when there is not.

This is the iPad split view and the tablet two-pane layout: below ScreenSize.compact only master is built and selecting a row is expected to push a page; from ScreenSize.medium up, detail fills the pane next to it and no navigation happens at all.

AdaptiveMasterDetail(
  master: ConversationList(onSelect: _select),
  detail: selected == null ? null : ConversationView(selected!),
  placeholder: const EmptyConversationState(),
)

The selection handler has to know which of the two modes is live, since pushing a page while the detail pane is visible would cover the pane it just filled. Ask isSplit:

void _select(Conversation c) {
  setState(() => selected = c);
  if (!AdaptiveMasterDetail.isSplit(context)) {
    Navigator.of(context).push(adaptivePageRoute(ConversationView(c)));
  }
}

Width buys a second pane, not larger type

The whole point of the layout is that a wide window shows two things at once instead of one thing blown up. Text inside either pane keeps the size it has on a phone — see AdaptiveTypography.

Measures its box, not the window

The decision comes from a LayoutBuilder, so the layout stays correct when it is itself inside a pane: an iPad in Slide Over, or a window the user dragged to a third of the screen, gets the single-pane treatment it should.