Transparency: isolation mode and opacity mask editing can't be entered from each other

Enter Isolation Mode worked while an opacity mask was edited, and Edit
Opacity Mask and View Opacity Mask worked in isolation mode, so each
mode replaced the other's isolated art. In the reference app, isolation
mode can't be entered while a mask is edited, and mask editing can't be
entered in isolation mode.

object.isolate and perspective.editText, which shows type in isolation
mode, are now off while an opacity mask is edited, and a call returns
an error that ends "stop editing the opacity mask first". In isolation
mode transparency.editOpacityMask and transparency.viewOpacityMask are
off, and transparency.makeOpacityMask refuses one target, an object or
a layer, which it would take into mask editing; a call returns an error
that ends "exit isolation mode first". Make Opacity Mask with two
objects still masks one with the other. Pattern editing mode does not
count as isolation mode for these commands.

Object > Perspective > Edit Text, the canvas context menu and the
Transparency panel's menu and Make Mask button follow the commands'
enablement. A click or Alt-click on the mask thumbnail in isolation
mode shows the error and does not edit the mask. While a mask is
edited, the Contextual Task Bar leaves out Isolate, and the Properties
panel's Quick Actions leave out Isolate Group.

Docs: docs/mcp.md, docs/target-app-parity.md.

crates/engine/src/cmd/maskedit.rs

@@ -5,7 +5,9 @@
 //! object updates live; the renderer doesn't paint the editing layer (the mask is seen through its
 //! effect, as in the reference app). Leaving the mode removes the layer, and saving or exporting
 //! never writes it ([`Document::without_edit_modes`]). Meanwhile the Transparency panel's commands
-//! act on the masked object (see `opacitymask::transparency_targets`).
+//! act on the masked object (see `opacitymask::transparency_targets`). Mask editing is not entered
+//! in isolation mode ([`outside_isolation_mode`]), and isolation mode is not entered while a mask is
+//! edited ([`outside_mask_editing`]).
 
 use std::sync::Arc;
 
@@ -27,8 +29,8 @@ pub fn specs() -> Vec<CommandSpec> {
             "Edit Opacity Mask",
             ["Window", "Transparency"],
             None,
-            "{id?} edit the mask art of `id` (default: the first selected object with a mask) in place, as clicking the mask thumbnail in the Transparency panel does → {layer}",
-            has_doc,
+            "{id?} edit the mask art of `id` (default: the first selected object with a mask) in place, as clicking the mask thumbnail in the Transparency panel does; refused in isolation mode → {layer}",
+            outside_isolation_mode,
             enter
         ),
         cmd!(
@@ -45,13 +47,33 @@ pub fn specs() -> Vec<CommandSpec> {
             "View Opacity Mask",
             ["Window", "Transparency"],
             None,
-            "{on?: bool (default: toggle), id?} show only the opacity mask of `id` (default: the object whose mask is edited, else the first selected object with a mask) on the canvas, as greyscale coverage, and edit it, as Alt-clicking the mask thumbnail does; off shows the artwork again (still editing) → {on, id}",
-            has_doc,
+            "{on?: bool (default: toggle), id?} show only the opacity mask of `id` (default: the object whose mask is edited, else the first selected object with a mask) on the canvas, as greyscale coverage, and edit it, as Alt-clicking the mask thumbnail does; off shows the artwork again (still editing); refused in isolation mode → {on, id}",
+            outside_isolation_mode,
             view_mask
         ),
     ]
 }
 
+/// Is the document in isolation mode? Pattern editing mode and opacity mask editing isolate their
+/// art too, and do not count as isolation mode here.
+pub(crate) fn in_isolation_mode(st: &crate::DocState) -> bool {
+    st.isolation.is_some() && st.doc.pattern_edit.is_none() && st.doc.mask_edit.is_none()
+}
+
+/// A document is open and it is not in isolation mode ([`in_isolation_mode`]): opacity mask
+/// editing is not entered in isolation mode.
+pub(crate) fn outside_isolation_mode(s: &Session) -> std::result::Result<(), String> {
+    let st = s.active().ok_or("no document open")?;
+    if in_isolation_mode(st) { Err("exit isolation mode first".into()) } else { Ok(()) }
+}
+
+/// A document is open and no opacity mask is being edited: isolation mode is not entered while
+/// an opacity mask is edited.
+pub(crate) fn outside_mask_editing(s: &Session) -> std::result::Result<(), String> {
+    let st = s.active().ok_or("no document open")?;
+    if st.doc.mask_edit.is_some() { Err("stop editing the opacity mask first".into()) } else { Ok(()) }
+}
+
 /// Mask art made of `children`: a plain group, so the mask keeps one art node. Editing spreads it
 /// back out onto the editing layer.
 pub(crate) fn mask_group(children: Vec<Arc<Node>>) -> Node {