Window: with Open Documents As Tabs cleared, new documents open in windows

Preferences > User Interface > Open Documents As Tabs was stored but did
nothing. With the box cleared, each document opened or created afterwards
opens in a floating window of its own, at the next cascade place, as
Window > Arrange > Float in Window floats it. Documents already open, and
documents open when the app starts, keep their tabs; checking the box
again leaves floating documents in their windows.

The engine records the preference on each document as it opens
(DocState::opened_as_tab), and the UI finds new documents by uid, so a
command batch that closes and opens documents, or changes the preference
between opening them, places each one as the preference was when it
opened.

crates/engine/src/doc_state.rs

@@ -52,6 +52,9 @@ pub struct DocState {
     pub recalled_selection: RecalledSelection,
     /// Process-unique id of this open document (tab indices shift when tabs close).
     pub uid: u64,
+    /// Preferences › User Interface › Open Documents As Tabs when the document opened: cleared,
+    /// the UI opens it in a floating window of its own.
+    pub opened_as_tab: bool,
     /// View Opacity Mask (Alt-click the mask thumbnail): the masked object whose mask the canvas
     /// shows alone, in greyscale, while that mask is edited (see [`DocState::shown_mask`]).
     pub mask_view: Option<NodeId>,
@@ -255,6 +258,7 @@ impl DocState {
             last_selection_cmd: None,
             recalled_selection: RecalledSelection::default(),
             uid: NEXT_DOC_UID.fetch_add(1, std::sync::atomic::Ordering::Relaxed),
+            opened_as_tab: true,
             mask_view: None,
             transparency_grid,
             format: "vectorcraft",

crates/engine/src/session/documents.rs

@@ -209,6 +209,7 @@ impl Session {
         self.reset_tool_for_doc_switch();
         let mut st = DocState::new(doc, path);
         st.history.limit = self.prefs.history_states as usize;
+        st.opened_as_tab = self.prefs.open_documents_as_tabs;
         self.docs.push(st);
         let i = self.docs.len() - 1;
         self.active = Some(i);
@@ -239,6 +240,7 @@ impl Session {
         st.history.limit = old.history.limit;
         // Same open document (caches keyed by uid stay valid); a new revision redraws it.
         st.uid = old.uid;
+        st.opened_as_tab = old.opened_as_tab;
         st.revision = old.revision + 1;
         st.format = old.format;
         st.save_options = old.save_options.clone();