diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index b1c216d3a..4af7f0a98 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -467,13 +467,13 @@ jobs:
 
     - name: no_std lint
       run: |
-        cargo clippy --target=thumbv7em-none-eabihf --no-default-features -p all-is-cubes -p all-is-cubes-render -p all-is-cubes-mesh
+        cargo clippy --target=thumbv7em-none-eabihf --no-default-features -p all-is-cubes -p all-is-cubes-render -p all-is-cubes-ui -p all-is-cubes-mesh
 
     - name: no_std build
       # This is `cargo build`, not `cargo check`, because `cargo check` won't detect problems like
       # use of undefined linker symbols. Not sure if that matters.
       run: |
-        cargo build --target=thumbv7em-none-eabihf --no-default-features -p all-is-cubes -p all-is-cubes-render -p all-is-cubes-mesh
+        cargo build --target=thumbv7em-none-eabihf --no-default-features -p all-is-cubes -p all-is-cubes-render -p all-is-cubes-ui -p all-is-cubes-mesh
 
   fuzz:
     # Don't spend time on fuzzing if the build failed indicating the code is bad other ways
diff --git a/Cargo.lock b/Cargo.lock
index d59c4123b..c516946b2 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -312,6 +312,7 @@ dependencies = [
  "futures-util",
  "indoc",
  "log",
+ "num-traits",
  "pretty_assertions",
  "scopeguard",
  "sync_wrapper 1.0.1",
diff --git a/all-is-cubes-content/Cargo.toml b/all-is-cubes-content/Cargo.toml
index 4acb7df97..eadd2afef 100644
--- a/all-is-cubes-content/Cargo.toml
+++ b/all-is-cubes-content/Cargo.toml
@@ -12,7 +12,7 @@ categories = ["games"]
 keywords = ["all-is-cubes"]
 
 [package.metadata.docs.rs]
-features = ["_full-documentation"]
+features = ["arbitrary"]
 
 [lib]
 # Disable running as benchmark so that the default doesn't interfere with Criterion usage.
@@ -25,12 +25,10 @@ harness = false
 [features]
 # Adds `UniverseTemplate::Random`.
 arbitrary = ["dep:arbitrary", "all-is-cubes/arbitrary"]
-# Internal use only. Used to ensure that docs.rs builds have complete information;
-# in particular, `all-is-cubes/std` would otherwise be missing, making types appear `!Sync`.
-_full-documentation = ["all-is-cubes/std", "arbitrary"]
 
 [dependencies]
-all-is-cubes = { workspace = true }
+# std feature is required because this library does not yet participate in `maybe_sync`.
+all-is-cubes = { workspace = true, features = ["std"] }
 all-is-cubes-ui = { workspace = true }
 arbitrary = { workspace = true, optional = true }
 exhaust = { workspace = true }
@@ -39,9 +37,9 @@ hashbrown = { workspace = true }
 itertools = { workspace = true }
 log = { workspace = true }
 macro_rules_attribute = { workspace = true }
-noise = { workspace = true }
+noise = { workspace = true } # TODO: not no_std compatible (fixable)
 paste = { workspace = true }
-petgraph = { workspace = true }
+petgraph = { workspace = true } # TODO: not no_std compatible (might become so someday)
 rand = { workspace = true }
 rand_xoshiro = { workspace = true }
 strum = { workspace = true, features = ["derive"] }
diff --git a/all-is-cubes-content/src/lib.rs b/all-is-cubes-content/src/lib.rs
index 2c57f41d4..5413ebb63 100644
--- a/all-is-cubes-content/src/lib.rs
+++ b/all-is-cubes-content/src/lib.rs
@@ -7,6 +7,10 @@
 //! provide [`UniverseTemplate`]; other items should be assumed not particularly
 //! stable.
 
+// This crate is *almost* `no_std` compatible; critically, some dependencies are not.
+// See comments in `Cargo.toml` for details.
+// For now, the code is just in a state of “reveal how close it is”, hence using `core` and
+// `alloc` imports only.
 #![no_std]
 //
 // Crate-specific lint settings. (General settings can be found in the workspace manifest.)
diff --git a/all-is-cubes-desktop/Cargo.toml b/all-is-cubes-desktop/Cargo.toml
index 168637f4e..574ba66e0 100644
--- a/all-is-cubes-desktop/Cargo.toml
+++ b/all-is-cubes-desktop/Cargo.toml
@@ -70,7 +70,7 @@ all-is-cubes-mesh = { workspace = true, optional = true, features = ["dynamic"]
 all-is-cubes-port = { workspace = true, features = ["import", "native"] }
 # TODO: make raytracer optional
 all-is-cubes-render = { workspace = true, features = ["raytracer"] }
-all-is-cubes-ui = { workspace = true }
+all-is-cubes-ui = { workspace = true, features = ["session"] }
 anyhow = { workspace = true }
 cfg-if = { workspace = true }
 clap = { workspace = true }
diff --git a/all-is-cubes-port/Cargo.toml b/all-is-cubes-port/Cargo.toml
index aa94c7689..1dbc71a85 100644
--- a/all-is-cubes-port/Cargo.toml
+++ b/all-is-cubes-port/Cargo.toml
@@ -57,7 +57,7 @@ dot-vox = [
 
 [dependencies]
 # Non-optional generally required dependencies
-# need all-is-cubes/std because we have boxed Send futures
+# need all-is-cubes/std because we have boxed Send futures and errors
 all-is-cubes = { workspace = true, features = ["std"] }
 cfg-if = { workspace = true }
 futures-core = { workspace = true }
diff --git a/all-is-cubes-ui/Cargo.toml b/all-is-cubes-ui/Cargo.toml
index ed818496c..4e7c7e543 100644
--- a/all-is-cubes-ui/Cargo.toml
+++ b/all-is-cubes-ui/Cargo.toml
@@ -12,27 +12,41 @@ categories = ["games", "gui"]
 keywords = ["all-is-cubes", "voxel"]
 
 [package.metadata.docs.rs]
-features = []
+features = ["session"]
 
 [lib]
 # Disable running as benchmark so that the default doesn't interfere with Criterion usage.
 bench = false
 
+[features]
+# Enable the `apps` and `notification` modules, particularly including the `Session` type,
+# and require `std` support.
+session = [
+    "all-is-cubes/std",
+    "dep:async_fn_traits",
+    "dep:flume",
+    "dep:futures-core",
+    "dep:futures-task",
+    "dep:futures-util",
+    "dep:scopeguard",
+    "dep:sync_wrapper",
+]
+
 [dependencies]
-# TODO: Disable default features. Requires more `maybe_sync` work.
-all-is-cubes = { workspace = true, default-features = true }
-all-is-cubes-render = { workspace = true, default-features = true }
-async_fn_traits = { workspace = true }
+all-is-cubes = { workspace = true, default-features = false }
+all-is-cubes-render = { workspace = true, default-features = false }
+async_fn_traits = { workspace = true, optional = true }
 displaydoc = { workspace = true }
 exhaust = { workspace = true }
-flume = { workspace = true }
-futures-core = { workspace = true }
-futures-task = { workspace = true }
-futures-util = { workspace = true }
+flume = { workspace = true, optional = true }
+futures-core = { workspace = true, optional = true }
+futures-task = { workspace = true, optional = true }
+futures-util = { workspace = true, optional = true }
 indoc = { workspace = true }
 log = { workspace = true }
-scopeguard = { workspace = true }
-sync_wrapper = { workspace = true }
+num-traits = { workspace = true }
+scopeguard = { workspace = true, optional = true }
+sync_wrapper = { workspace = true, optional = true }
 
 [dev-dependencies]
 futures-channel = { workspace = true }
diff --git a/all-is-cubes-ui/src/apps/input.rs b/all-is-cubes-ui/src/apps/input.rs
index 7369d9aab..5102a4d97 100644
--- a/all-is-cubes-ui/src/apps/input.rs
+++ b/all-is-cubes-ui/src/apps/input.rs
@@ -3,6 +3,8 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
+
+use alloc::vec::Vec;
 use core::time::Duration;
 use std::collections::{HashMap, HashSet};
 
diff --git a/all-is-cubes-ui/src/apps/session.rs b/all-is-cubes-ui/src/apps/session.rs
index 139bf5583..de95ea1d4 100644
--- a/all-is-cubes-ui/src/apps/session.rs
+++ b/all-is-cubes-ui/src/apps/session.rs
@@ -3,6 +3,8 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
+use alloc::boxed::Box;
+use alloc::string::ToString as _;
 use alloc::sync::{Arc, Weak};
 use core::fmt;
 use core::future::Future;
diff --git a/all-is-cubes-ui/src/editor.rs b/all-is-cubes-ui/src/editor.rs
index fe6ea5d59..64fc8bd1a 100644
--- a/all-is-cubes-ui/src/editor.rs
+++ b/all-is-cubes-ui/src/editor.rs
@@ -1,13 +1,15 @@
 //! VUI components related to allowing the user to inspect universe contents.
 
-use all_is_cubes::universe::Handle;
 use alloc::sync::Arc;
+use alloc::vec;
+use alloc::vec::Vec;
 
 use all_is_cubes::arcstr::{self, literal, ArcStr};
 use all_is_cubes::block::{self, Block};
 use all_is_cubes::character::Cursor;
 use all_is_cubes::euclid::size3;
 use all_is_cubes::math::Face6;
+use all_is_cubes::universe::Handle;
 
 use crate::ui_content::hud::HudInputs;
 use crate::ui_content::pages::back_button;
diff --git a/all-is-cubes-ui/src/inv_watch.rs b/all-is-cubes-ui/src/inv_watch.rs
index 9fb90ed0f..9a025e636 100644
--- a/all-is-cubes-ui/src/inv_watch.rs
+++ b/all-is-cubes-ui/src/inv_watch.rs
@@ -4,7 +4,7 @@
 //! "derived information from a `ListenableSource` that requires computation" and should become
 //! general code that handles the re-listening problem.
 
-use std::sync::Arc;
+use alloc::sync::Arc;
 
 use all_is_cubes::character::{Character, CharacterChange};
 use all_is_cubes::inv::{Inventory, TOOL_SELECTIONS};
diff --git a/all-is-cubes-ui/src/lib.rs b/all-is-cubes-ui/src/lib.rs
index 18280a0c9..8964046a4 100644
--- a/all-is-cubes-ui/src/lib.rs
+++ b/all-is-cubes-ui/src/lib.rs
@@ -18,21 +18,29 @@
 //! [`Space`]: all_is_cubes::space::Space
 //! [`Universe`]: all_is_cubes::universe::Universe
 
-// This crate is not `no_std` because it currently uses `std::sync::mpsc`.
-
+#![no_std]
 // Crate-specific lint settings. (General settings can be found in the workspace manifest.)
 #![forbid(unsafe_code)]
 
 extern crate alloc;
 
+#[cfg(any(test, feature = "session"))]
+#[macro_use]
+extern crate std;
+
+#[cfg(feature = "session")]
 mod editor;
+#[cfg(feature = "session")]
 mod inv_watch;
 
+#[cfg(feature = "session")]
 pub mod apps;
 
 pub mod logo;
 
+#[cfg(feature = "session")]
 mod ui_content;
+#[cfg(feature = "session")]
 pub use ui_content::notification;
 
 pub mod vui;
diff --git a/all-is-cubes-ui/src/ui_content/notification.rs b/all-is-cubes-ui/src/ui_content/notification.rs
index 696ba6a33..9cca6a527 100644
--- a/all-is-cubes-ui/src/ui_content/notification.rs
+++ b/all-is-cubes-ui/src/ui_content/notification.rs
@@ -1,6 +1,7 @@
 //! Types used to create notifications displayed to the user.
 
 use alloc::sync::{Arc, Weak};
+use alloc::vec::Vec;
 use std::sync::Mutex;
 
 use all_is_cubes::listen;
diff --git a/all-is-cubes-ui/src/ui_content/options.rs b/all-is-cubes-ui/src/ui_content/options.rs
index 377de87bd..36cf1cef1 100644
--- a/all-is-cubes-ui/src/ui_content/options.rs
+++ b/all-is-cubes-ui/src/ui_content/options.rs
@@ -1,4 +1,6 @@
+use alloc::boxed::Box;
 use alloc::sync::Arc;
+use alloc::vec::Vec;
 use core::fmt;
 
 use all_is_cubes::arcstr::{self, literal};
diff --git a/all-is-cubes-ui/src/ui_content/pages.rs b/all-is-cubes-ui/src/ui_content/pages.rs
index 44fbd940c..b1984bb34 100644
--- a/all-is-cubes-ui/src/ui_content/pages.rs
+++ b/all-is-cubes-ui/src/ui_content/pages.rs
@@ -1,5 +1,6 @@
 //! Contents of various UI pages.
 
+use alloc::string::String;
 use alloc::sync::Arc;
 
 use all_is_cubes::arcstr::{literal, ArcStr};
diff --git a/all-is-cubes-ui/src/ui_content/vui_manager.rs b/all-is-cubes-ui/src/ui_content/vui_manager.rs
index 83eded86a..3f080ae60 100644
--- a/all-is-cubes-ui/src/ui_content/vui_manager.rs
+++ b/all-is-cubes-ui/src/ui_content/vui_manager.rs
@@ -1,3 +1,4 @@
+use alloc::string::{String, ToString as _};
 use alloc::sync::Arc;
 use core::future::Future;
 use flume::TryRecvError;
diff --git a/all-is-cubes-ui/src/vui/layout.rs b/all-is-cubes-ui/src/vui/layout.rs
index 55432e4bd..1b356bd89 100644
--- a/all-is-cubes-ui/src/vui/layout.rs
+++ b/all-is-cubes-ui/src/vui/layout.rs
@@ -3,8 +3,11 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
+use alloc::boxed::Box;
 use alloc::rc::Rc;
 use alloc::sync::Arc;
+use alloc::vec;
+use alloc::vec::Vec;
 use core::fmt;
 
 use all_is_cubes::euclid::{self, size3, Size3D, Vector3D};
diff --git a/all-is-cubes-ui/src/vui/mod.rs b/all-is-cubes-ui/src/vui/mod.rs
index aedf378ce..01b77de22 100644
--- a/all-is-cubes-ui/src/vui/mod.rs
+++ b/all-is-cubes-ui/src/vui/mod.rs
@@ -6,10 +6,13 @@
 
 #[doc(hidden)] // public for use by test-renderers only
 pub mod blocks;
+#[cfg(feature = "session")]
 pub(crate) use blocks::UiBlocks;
 mod layout;
 pub use layout::*;
+#[cfg(feature = "session")]
 mod page;
+#[cfg(feature = "session")]
 pub(crate) use page::*;
 mod widget_trait;
 pub use widget_trait::*;
diff --git a/all-is-cubes-ui/src/vui/page.rs b/all-is-cubes-ui/src/vui/page.rs
index 84597ca78..9eea84931 100644
--- a/all-is-cubes-ui/src/vui/page.rs
+++ b/all-is-cubes-ui/src/vui/page.rs
@@ -1,4 +1,5 @@
 use alloc::sync::Arc;
+use alloc::vec;
 
 use all_is_cubes::arcstr::ArcStr;
 use all_is_cubes::block::{text, AIR};
@@ -336,6 +337,7 @@ pub(crate) mod parts {
 #[cfg(test)]
 mod tests {
     use super::*;
+    use alloc::vec::Vec;
 
     #[test]
     fn ui_size() {
diff --git a/all-is-cubes-ui/src/vui/widget_trait.rs b/all-is-cubes-ui/src/vui/widget_trait.rs
index 32b3b7a6f..d502cf91f 100644
--- a/all-is-cubes-ui/src/vui/widget_trait.rs
+++ b/all-is-cubes-ui/src/vui/widget_trait.rs
@@ -1,17 +1,17 @@
 //! UI [`Widget`] trait and related glue.
 
 use all_is_cubes::time::Tick;
+use alloc::boxed::Box;
 use alloc::sync::Arc;
 use core::error::Error;
 use core::fmt::Debug;
-use std::sync::Mutex;
 
 use all_is_cubes::behavior::{self, Behavior};
 use all_is_cubes::math::GridAab;
 use all_is_cubes::space::{self, Space, SpaceTransaction};
 use all_is_cubes::transaction::{self, Merge as _};
 use all_is_cubes::universe::{HandleVisitor, UniverseTransaction, VisitHandles};
-use all_is_cubes::util::maybe_sync::SendSyncIfStd;
+use all_is_cubes::util::maybe_sync::{self, SendSyncIfStd};
 
 // reused for WidgetController
 pub use all_is_cubes::behavior::Then;
@@ -113,7 +113,9 @@ pub type StepSuccess = (WidgetTransaction, Then);
 /// Error return of [`WidgetController::step()`].
 ///
 /// TODO: This should become a more specific error type.
-pub type StepError = Box<dyn Error + Send + Sync>;
+//---
+// This is a type alias to cope with the maybe-Sync kludge without exposing that everywhere.
+pub type StepError = maybe_sync::BoxError;
 
 impl WidgetController for Box<dyn WidgetController> {
     fn step(&mut self, context: &WidgetContext<'_>) -> Result<StepSuccess, StepError> {
@@ -137,7 +139,7 @@ impl WidgetController for Box<dyn WidgetController> {
 pub(super) struct WidgetBehavior {
     /// Original widget -- not used directly but for error reporting
     widget: Positioned<Arc<dyn Widget>>,
-    controller: Mutex<Box<dyn WidgetController>>,
+    controller: maybe_sync::Mutex<Box<dyn WidgetController>>,
 }
 
 impl WidgetBehavior {
@@ -164,7 +166,7 @@ impl WidgetBehavior {
             space::SpaceBehaviorAttachment::new(widget.position.bounds),
             Arc::new(WidgetBehavior {
                 widget,
-                controller: Mutex::new(controller),
+                controller: maybe_sync::Mutex::new(controller),
             }),
         );
         init_txn
@@ -325,11 +327,11 @@ pub(crate) fn instantiate_widget<W: Widget + 'static>(
 #[cfg(test)]
 mod tests {
     use super::*;
-    use all_is_cubes::util::assert_send_sync;
+    use all_is_cubes::util::assert_conditional_send_sync;
 
     #[test]
     fn error_is_send_sync() {
-        assert_send_sync::<InstallVuiError>()
+        assert_conditional_send_sync::<InstallVuiError>()
     }
 
     fn _assert_widget_trait_is_object_safe(_: &dyn Widget) {}
diff --git a/all-is-cubes-ui/src/vui/widgets/button.rs b/all-is-cubes-ui/src/vui/widgets/button.rs
index 9f4c6be9d..a819119bf 100644
--- a/all-is-cubes-ui/src/vui/widgets/button.rs
+++ b/all-is-cubes-ui/src/vui/widgets/button.rs
@@ -3,6 +3,9 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
+use alloc::borrow::ToOwned as _;
+use alloc::boxed::Box;
+use alloc::format;
 use alloc::sync::Arc;
 use core::fmt;
 use core::hash::Hash;
@@ -549,7 +552,7 @@ mod theme {
     /// Build a [`Block`] for [`ButtonBase`].
     pub fn common_block(space: Handle<Space>, name: &str) -> Block {
         Block::builder()
-            .display_name(name.to_string())
+            .display_name(name)
             .voxels_handle(MULTI_RESOLUTION, space)
             .animation_hint(block::AnimationHint::replacement(
                 block::AnimationChange::Shape,
diff --git a/all-is-cubes-ui/src/vui/widgets/crosshair.rs b/all-is-cubes-ui/src/vui/widgets/crosshair.rs
index 38cf80fbf..4535589c0 100644
--- a/all-is-cubes-ui/src/vui/widgets/crosshair.rs
+++ b/all-is-cubes-ui/src/vui/widgets/crosshair.rs
@@ -1,3 +1,4 @@
+use alloc::boxed::Box;
 use alloc::sync::Arc;
 
 use all_is_cubes::block::{Block, AIR};
diff --git a/all-is-cubes-ui/src/vui/widgets/debug.rs b/all-is-cubes-ui/src/vui/widgets/debug.rs
index 28fe48a85..290cd0fde 100644
--- a/all-is-cubes-ui/src/vui/widgets/debug.rs
+++ b/all-is-cubes-ui/src/vui/widgets/debug.rs
@@ -1,3 +1,4 @@
+use alloc::boxed::Box;
 use alloc::sync::Arc;
 
 use all_is_cubes::block::{self, text, Block, Resolution::R64};
diff --git a/all-is-cubes-ui/src/vui/widgets/frame.rs b/all-is-cubes-ui/src/vui/widgets/frame.rs
index cd0546ea3..f8e53f41d 100644
--- a/all-is-cubes-ui/src/vui/widgets/frame.rs
+++ b/all-is-cubes-ui/src/vui/widgets/frame.rs
@@ -1,4 +1,6 @@
+use alloc::boxed::Box;
 use alloc::sync::Arc;
+use alloc::vec;
 
 use all_is_cubes::block::Block;
 use all_is_cubes::euclid::size3;
diff --git a/all-is-cubes-ui/src/vui/widgets/mod.rs b/all-is-cubes-ui/src/vui/widgets/mod.rs
index 03eb1b379..6efe9edb9 100644
--- a/all-is-cubes-ui/src/vui/widgets/mod.rs
+++ b/all-is-cubes-ui/src/vui/widgets/mod.rs
@@ -1,5 +1,6 @@
 //! Specific UI widgets.
 
+use alloc::boxed::Box;
 use alloc::sync::Arc;
 
 use all_is_cubes::block::Block;
@@ -8,7 +9,9 @@ use all_is_cubes::space::SpaceTransaction;
 
 use crate::vui;
 
+#[cfg(feature = "session")]
 mod crosshair;
+#[cfg(feature = "session")]
 pub(crate) use crosshair::*;
 mod frame;
 pub use frame::*;
@@ -22,9 +25,13 @@ mod progress_bar;
 pub use progress_bar::*;
 mod theme;
 pub use theme::*;
+#[cfg(feature = "session")]
 mod toolbar;
+#[cfg(feature = "session")]
 pub(crate) use toolbar::*;
+#[cfg(feature = "session")]
 mod tooltip;
+#[cfg(feature = "session")]
 pub(crate) use tooltip::*;
 mod voxels;
 pub use voxels::*;
diff --git a/all-is-cubes-ui/src/vui/widgets/progress_bar.rs b/all-is-cubes-ui/src/vui/widgets/progress_bar.rs
index 8496f4885..1f230c427 100644
--- a/all-is-cubes-ui/src/vui/widgets/progress_bar.rs
+++ b/all-is-cubes-ui/src/vui/widgets/progress_bar.rs
@@ -3,7 +3,13 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
-use std::sync::Arc;
+use alloc::boxed::Box;
+use alloc::sync::Arc;
+
+/// Acts as polyfill for float methods
+#[cfg(not(feature = "session"))]
+#[allow(unused_imports)]
+use num_traits::float::FloatCore as _;
 
 use all_is_cubes::block::{self, Block, Composite, CompositeOperator, AIR};
 use all_is_cubes::color_block;
@@ -208,6 +214,8 @@ mod tests {
     use all_is_cubes::transaction::Transaction as _;
     use all_is_cubes::util::yield_progress_for_testing;
     use all_is_cubes::{transaction, universe};
+    use alloc::string::String;
+    use alloc::vec::Vec;
 
     #[tokio::test]
     async fn progress_output() {
diff --git a/all-is-cubes-ui/src/vui/widgets/text.rs b/all-is-cubes-ui/src/vui/widgets/text.rs
index 1a9d5710d..c524979f4 100644
--- a/all-is-cubes-ui/src/vui/widgets/text.rs
+++ b/all-is-cubes-ui/src/vui/widgets/text.rs
@@ -3,6 +3,7 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
+use alloc::boxed::Box;
 use alloc::sync::Arc;
 
 use all_is_cubes::arcstr::ArcStr;
@@ -114,6 +115,7 @@ impl Label {
     /// Constructs a [`Label`] that draws the given text, with a specified font.
     //---
     // TODO: undecided what's a good API
+    #[cfg_attr(not(feature = "session"), expect(dead_code))]
     pub(crate) fn with_font(
         string: ArcStr,
         font: text::Font,
diff --git a/all-is-cubes-ui/src/vui/widgets/theme.rs b/all-is-cubes-ui/src/vui/widgets/theme.rs
index 2228a0d95..6380b6dfb 100644
--- a/all-is-cubes-ui/src/vui/widgets/theme.rs
+++ b/all-is-cubes-ui/src/vui/widgets/theme.rs
@@ -3,8 +3,9 @@
     reason = "module is private; https://github.com/rust-lang/rust-clippy/issues/8524"
 )]
 
+use alloc::format;
+use alloc::sync::Arc;
 use core::fmt;
-use std::sync::Arc;
 
 use exhaust::Exhaust;
 
@@ -207,16 +208,18 @@ impl WidgetBlocks {
                 }
 
                 WidgetBlocks::ProgressBar { full } => {
+                    let image = if full {
+                        include_image!("theme/progress-bar-full.png")
+                    } else {
+                        include_image!("theme/progress-bar-empty.png")
+                    };
+
                     Block::builder()
                         .display_name(format! {"Progress Bar {}", if full {"Full"} else {"Empty"}})
                         .voxels_handle(
                             R64, // 16 res × 4 tiles
                             txn.insert_anonymous(space_from_image(
-                                if full {
-                                    include_image!("theme/progress-bar-full.png")
-                                } else {
-                                    include_image!("theme/progress-bar-empty.png")
-                                },
+                                image,
                                 GridRotation::IDENTITY,
                                 &default_srgb,
                             )?),
diff --git a/all-is-cubes-ui/src/vui/widgets/toolbar.rs b/all-is-cubes-ui/src/vui/widgets/toolbar.rs
index b53f8156b..c8945811c 100644
--- a/all-is-cubes-ui/src/vui/widgets/toolbar.rs
+++ b/all-is-cubes-ui/src/vui/widgets/toolbar.rs
@@ -1,3 +1,4 @@
+use alloc::boxed::Box;
 use alloc::sync::{Arc, Weak};
 use core::error::Error;
 use core::fmt;
diff --git a/all-is-cubes-ui/src/vui/widgets/tooltip.rs b/all-is-cubes-ui/src/vui/widgets/tooltip.rs
index 9ddabc9d4..eb3ba1b74 100644
--- a/all-is-cubes-ui/src/vui/widgets/tooltip.rs
+++ b/all-is-cubes-ui/src/vui/widgets/tooltip.rs
@@ -1,3 +1,4 @@
+use alloc::boxed::Box;
 use alloc::sync::Arc;
 use std::sync::Mutex;
 
diff --git a/all-is-cubes-ui/src/vui/widgets/voxels.rs b/all-is-cubes-ui/src/vui/widgets/voxels.rs
index 53ba816ab..47a37ebb7 100644
--- a/all-is-cubes-ui/src/vui/widgets/voxels.rs
+++ b/all-is-cubes-ui/src/vui/widgets/voxels.rs
@@ -1,4 +1,6 @@
+use alloc::boxed::Box;
 use alloc::sync::Arc;
+use alloc::vec::Vec;
 
 use all_is_cubes::block::{self, Block, Primitive, Resolution};
 use all_is_cubes::euclid::Size3D;
diff --git a/all-is-cubes-wasm/Cargo.lock b/all-is-cubes-wasm/Cargo.lock
index ae819710f..d5ffddbeb 100644
--- a/all-is-cubes-wasm/Cargo.lock
+++ b/all-is-cubes-wasm/Cargo.lock
@@ -204,6 +204,7 @@ dependencies = [
  "futures-util",
  "indoc",
  "log",
+ "num-traits",
  "scopeguard",
  "sync_wrapper",
 ]
diff --git a/all-is-cubes-wasm/Cargo.toml b/all-is-cubes-wasm/Cargo.toml
index e2ff7eb68..1f91523c2 100644
--- a/all-is-cubes-wasm/Cargo.toml
+++ b/all-is-cubes-wasm/Cargo.toml
@@ -32,7 +32,7 @@ all-is-cubes-content = { path = "../all-is-cubes-content", version = "0.8.0" }
 all-is-cubes-gpu = { path = "../all-is-cubes-gpu", version = "0.8.0", features = ["wgpu"] }
 all-is-cubes-port = { path = "../all-is-cubes-port", version = "0.8.0", features = ["import", "all-formats"] }
 all-is-cubes-render = { path = "../all-is-cubes-render", version = "0.8.0" }
-all-is-cubes-ui = { path = "../all-is-cubes-ui", version = "0.8.0" }
+all-is-cubes-ui = { path = "../all-is-cubes-ui", version = "0.8.0", features = ["session"] }
 console_log = "1.0.0"
 console_error_panic_hook = "0.1.5"
 form_urlencoded = "1.0.1"
diff --git a/all-is-cubes/src/linking.rs b/all-is-cubes/src/linking.rs
index e49ef58e4..08e7d3201 100644
--- a/all-is-cubes/src/linking.rs
+++ b/all-is-cubes/src/linking.rs
@@ -22,6 +22,7 @@ use crate::block::{Block, BlockDef};
 use crate::space::{SetCubeError, SpaceTransaction};
 use crate::transaction::ExecuteError;
 use crate::universe::{Handle, InsertError, Name, Universe, UniverseTransaction};
+use crate::util::maybe_sync;
 use crate::util::YieldProgress;
 
 #[cfg(doc)]
@@ -395,10 +396,10 @@ impl From<ExecuteError<UniverseTransaction>> for GenError {
 #[non_exhaustive]
 pub enum InGenError {
     /// Generic error container for unusual situations.
-    Other(Box<dyn Error + Send + Sync>),
+    Other(maybe_sync::BoxError),
 
     /// Something else needed to be generated and that failed.
-    Gen(Box<GenError>),
+    Gen(Box<GenError>), // boxed due to being a recursive type
 
     /// Failed to insert the generated items in the [`Universe`].
     Insert(InsertError),
@@ -420,7 +421,7 @@ pub enum InGenError {
 impl InGenError {
     /// Convert an arbitrary error to `InGenError`.
     #[cfg_attr(not(feature = "std"), doc(hidden))]
-    pub fn other<E: Error + Send + Sync + 'static>(error: E) -> Self {
+    pub fn other<E: Error + maybe_sync::SendSyncIfStd + 'static>(error: E) -> Self {
         Self::Other(Box::new(error))
     }
 }
@@ -492,7 +493,7 @@ mod tests {
     use crate::content::make_some_blocks;
     use crate::math::GridAab;
     use crate::transaction::Transactional as _;
-    use crate::util::assert_send_sync;
+    use crate::util::assert_conditional_send_sync;
 
     #[derive(Exhaust, Clone, Debug, Eq, Hash, PartialEq)]
     enum Key {
@@ -570,8 +571,8 @@ mod tests {
 
     #[test]
     fn errors_are_send_sync() {
-        assert_send_sync::<GenError>();
-        assert_send_sync::<InGenError>();
+        assert_conditional_send_sync::<GenError>();
+        assert_conditional_send_sync::<InGenError>();
     }
 
     #[test]
diff --git a/all-is-cubes/src/util/maybe_sync.rs b/all-is-cubes/src/util/maybe_sync.rs
index 8c5a6cb27..4539791d2 100644
--- a/all-is-cubes/src/util/maybe_sync.rs
+++ b/all-is-cubes/src/util/maybe_sync.rs
@@ -1,24 +1,38 @@
+use alloc::boxed::Box;
+use core::error::Error;
 use core::{fmt, ops};
 
 cfg_if::cfg_if! {
     if #[cfg(feature = "std")] {
         pub trait SendSyncIfStd: Send + Sync {}
         impl<T: Send + Sync> SendSyncIfStd for T {}
+
+        /// This type alias for a boxed [`Future`] requires `Send` if the `std` feature is
+        /// enabled.
+        pub(crate) type MaybeLocalBoxFuture<'a, T> = futures_core::future::BoxFuture<'a, T>;
+
+        /// This type alias for a boxed [`Error`] requires `Send + Sync` if the `std` feature is
+        /// enabled.
+        #[doc(hidden)]
+        pub type BoxError = Box<dyn Error + Send + Sync>;
     } else {
         pub trait SendSyncIfStd {}
         impl<T> SendSyncIfStd for T {}
+
+        /// This type alias for a boxed [`Future`] requires `Send` if the `std` feature is
+        /// enabled.
+        pub(crate) type MaybeLocalBoxFuture<'a, T> = futures_core::future::LocalBoxFuture<'a, T>;
+
+        /// This type alias for a boxed [`Error`] requires `Send + Sync` if the `std` feature is
+        /// enabled.
+        #[doc(hidden)]
+        pub type BoxError = Box<dyn Error>;
     }
 }
 
 cfg_if::cfg_if! {
     if #[cfg(feature = "std")] {
-        /// This type alias for a boxed future requires `Sync` if the `std` feature is
-        /// enabled.
-        pub(crate) type MaybeLocalBoxFuture<'a, T> = futures_core::future::BoxFuture<'a, T>;
     } else {
-        /// This type alias for a boxed future requires `Sync` if the `std` feature is
-        /// enabled.
-        pub(crate) type MaybeLocalBoxFuture<'a, T> = futures_core::future::LocalBoxFuture<'a, T>;
     }
 }
 
@@ -31,9 +45,10 @@ cfg_if::cfg_if! {
 /// * This may or may not implement mutex poisoning.
 /// * This may or may not deadlock if locked again from the same thread.
 #[derive(Default)]
-pub(crate) struct Mutex<T>(InnerMutex<T>);
+pub struct Mutex<T>(InnerMutex<T>);
 
-pub(crate) struct MutexGuard<'a, T>(InnerMutexGuard<'a, T>);
+#[allow(missing_debug_implementations)]
+pub struct MutexGuard<'a, T>(InnerMutexGuard<'a, T>);
 
 /// Wrapper around [`core::cell::RefCell`] or [`std::sync::RwLock`] depending on whether
 /// the `std` feature is enabled.
@@ -196,11 +211,8 @@ impl<T> ops::DerefMut for RwLockWriteGuard<'_, T> {
     }
 }
 
-pub(crate) enum LockError<G> {
-    #[cfg_attr(
-        not(feature = "std"),
-        expect(dead_code, reason = "no poisoning from RefCell")
-    )]
+#[allow(clippy::exhaustive_enums)]
+pub enum LockError<G> {
     Poisoned(G),
 }
 
@@ -213,7 +225,7 @@ impl<G> LockError<G> {
     // }
 }
 
-impl<G> core::error::Error for LockError<G> {}
+impl<G> Error for LockError<G> {}
 
 impl<G> fmt::Display for LockError<G> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
@@ -240,7 +252,7 @@ pub(crate) enum TryLockError<G> {
     WouldBlock,
 }
 
-impl<G> core::error::Error for TryLockError<G> {}
+impl<G> Error for TryLockError<G> {}
 
 impl<G> fmt::Display for TryLockError<G> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
diff --git a/test-renderers/Cargo.toml b/test-renderers/Cargo.toml
index 58d2a1bdb..57593b498 100644
--- a/test-renderers/Cargo.toml
+++ b/test-renderers/Cargo.toml
@@ -70,7 +70,7 @@ all-is-cubes-gpu = { workspace = true, features = ["wgpu"] }
 # used only for gltf export+render tests
 all-is-cubes-port = { workspace = true, optional = true, features = ["export", "gltf"] }
 all-is-cubes-render = { workspace = true, features = ["raytracer", "std"] }
-all-is-cubes-ui = { workspace = true }
+all-is-cubes-ui = { workspace = true, features = ["session"] }
 async_fn_traits = { workspace = true }
 bytemuck = { workspace = true, optional = true }
 # Using clap for parsing test command-line arguments
diff --git a/tools/xtask/src/xtask.rs b/tools/xtask/src/xtask.rs
index 166e44742..4772695f7 100644
--- a/tools/xtask/src/xtask.rs
+++ b/tools/xtask/src/xtask.rs
@@ -648,13 +648,22 @@ fn do_for_all_packages(
                 }
 
                 {
-                    // Check `all-is-cubes` with default features disabled, because that's
-                    // more easily broken by accident (such as by introducing an unintended
+                    // Check no_std compatiblepackages with default features disabled, because
+                    // that's more easily broken by accident (such as by introducing an unintended
                     // `Send` bound) then our other features.
+                    //
+                    // TODO: Replace this one-off list of packages with something more centralized,
+                    // and shared with the CI that actually builds a no_std target.
                     let _t = CaptureTime::new(time_log, "check aic no_std");
                     cargo()
                         .arg(op.non_build_check_subcmd())
-                        .args(["--package=all-is-cubes", "--no-default-features"])
+                        .args([
+                            "--package=all-is-cubes",
+                            "--package=all-is-cubes-mesh",
+                            "--package=all-is-cubes-render",
+                            "--package=all-is-cubes-ui",
+                            "--no-default-features",
+                        ])
                         .run()?;
                 }
             }