Flatten tree

book/src/configuration/misc.md

@@ -227,6 +227,32 @@ instead.
 split-reuses-container = false  # default
 ```
 
+## Flatten Tree
+
+When a window is removed from a container (e.g. moved to another workspace or
+floated), or when pressing split-vertical/split-horizontal on a single window,
+the container may end up with only one child. By default the container is kept,
+resulting in a redundant nesting.
+
+The `flatten-tree` setting controls when such containers are automatically
+collapsed:
+
+`"never"`
+: Never collapse single-child containers (default).
+
+`"always"`
+: Collapse a single-child container both when its second-to-last child is
+  removed and when pressing split-vertical or split-horizontal on a sole
+  child.
+
+`"on-remove"`
+: Only collapse when the second-to-last child is removed (e.g. moved to
+  another workspace or floated).
+
+```toml
+flatten-tree = "on-remove"
+```
+
 ## Middle-Click Paste
 
 Controls whether middle-clicking pastes the primary selection. Changing this

book/src/control-center.md

@@ -479,6 +479,11 @@ Split Reuses Container
   direction instead of wrapping the window in a new container (see
   [Split Reuses Container](configuration/misc.md#split-reuses-container))
 
+Flatten Tree
+: Dropdown to select when single-child containers are flattened (`Never`,
+  `Always`, or `On Remove`; see
+  [Flatten Tree](configuration/misc.md#flatten-tree))
+
 Font
 : Text field for the main compositor font family
 

jay-config/src/_private/client.rs

@@ -23,6 +23,7 @@ use crate::_private::logging;
 use crate::Axis;
 use crate::ContainerTarget;
 use crate::Direction;
+use crate::FlattenTree;
 use crate::ModifiedKeySym;
 use crate::PciId;
 use crate::RelativeAxis;
@@ -1288,6 +1289,10 @@ impl ConfigClient {
         self.send(&ClientMessage::SetMiddleClickPasteEnabled { enabled });
     }
 
+    pub fn set_flatten_tree(&self, mode: FlattenTree) {
+        self.send(&ClientMessage::SetFlattenTree { mode });
+    }
+
     pub fn open_control_center(&self) {
         self.send(&ClientMessage::OpenControlCenter);
     }

jay-config/src/_private/ipc.rs

@@ -8,6 +8,7 @@ use crate::_private::WorkspaceShowOpV2;
 use crate::Axis;
 use crate::ContainerTarget;
 use crate::Direction;
+use crate::FlattenTree;
 use crate::PciId;
 use crate::RelativeAxis;
 use crate::Workspace;
@@ -1084,6 +1085,9 @@ pub enum ClientMessage<'a> {
         target: ContainerTarget,
         axis: RelativeAxis,
     },
+    SetFlattenTree {
+        mode: FlattenTree,
+    },
 }
 
 #[derive(Serialize, Deserialize, Debug)]

jay-config/src/lib.rs

@@ -87,6 +87,20 @@ pub enum Direction {
     Right,
 }
 
+/// Configures how single-child containers are flattened.
+#[derive(Serialize, Deserialize, Copy, Clone, Debug, Hash, Eq, PartialEq)]
+#[non_exhaustive]
+pub enum FlattenTree {
+    /// Never flatten single-child containers.
+    Never,
+    /// Always flatten single-child containers, including when pressing
+    /// split-vertical or split-horizontal on a sole child.
+    Always,
+    /// Only flatten when the second-to-last child is removed from a
+    /// container.
+    OnRemove,
+}
+
 /// A planar axis.
 #[derive(Serialize, Deserialize, Copy, Clone, Debug, Hash, Eq, PartialEq)]
 pub enum Axis {
@@ -597,6 +611,13 @@ pub fn set_middle_click_paste_enabled(enabled: bool) {
     get!().set_middle_click_paste_enabled(enabled);
 }
 
+/// Sets the tree flattening mode.
+///
+/// The default is [`FlattenTree::Never`].
+pub fn set_flatten_tree(mode: FlattenTree) {
+    get!().set_flatten_tree(mode);
+}
+
 /// Opens the control center.
 pub fn open_control_center() {
     get!().open_control_center();

src/compositor.rs

@@ -440,6 +440,7 @@ fn start_compositor2(
         color_management_enabled: Cell::new(false),
         color_manager,
         float_above_fullscreen: Cell::new(false),
+        flatten_tree: Default::default(),
         icons: Default::default(),
         show_pin_icon: Cell::new(false),
         cl_matcher_manager: Rc::new(ClMatcherManager::new(&crit_ids)),

src/config/handler.rs

@@ -99,6 +99,7 @@ use jay_config::_private::serialize_server_message;
 use jay_config::Axis;
 use jay_config::ContainerTarget as ConfigContainerTarget;
 use jay_config::Direction;
+use jay_config::FlattenTree as ConfigFlattenTree;
 use jay_config::RelativeAxis as ConfigRelativeAxis;
 use jay_config::Workspace;
 use jay_config::WorkspaceKind;
@@ -2943,6 +2944,14 @@ impl ConfigProxyHandler {
         self.state.set_primary_selection_enabled(enabled);
     }
 
+    fn handle_set_flatten_tree(&self, mode: ConfigFlattenTree) -> Result<(), CphError> {
+        let Ok(mode) = mode.try_into() else {
+            return Err(CphError::UnknownFlattenTree(mode));
+        };
+        self.state.set_flatten_tree(mode);
+        Ok(())
+    }
+
     fn handle_seat_create_mark(&self, seat: Seat, kc: Option<u32>) -> Result<(), CphError> {
         let seat = self.get_seat(seat)?;
         if let Some(kc) = kc {
@@ -4056,6 +4065,9 @@ impl ConfigProxyHandler {
             ClientMessage::SetMiddleClickPasteEnabled { enabled } => {
                 self.handle_set_middle_click_paste_enabled(enabled)
             }
+            ClientMessage::SetFlattenTree { mode } => {
+                self.handle_set_flatten_tree(mode).wrn("set_flatten_tree")?
+            }
             ClientMessage::SetWorkspaceDisplayOrder { order } => {
                 self.handle_set_workspace_display_order(order)
             }
@@ -4485,6 +4497,8 @@ enum CphError {
     UnknownContainerTarget(ConfigContainerTarget),
     #[error("Tried to use an unknown relative axis: {0:?}")]
     UnknownRelativeAxis(ConfigRelativeAxis),
+    #[error("Unknown flatten tree mode {0:?}")]
+    UnknownFlattenTree(ConfigFlattenTree),
 }
 
 trait WithRequestName {

src/control_center/cc_look_and_feel.rs

@@ -3,6 +3,7 @@ use crate::control_center::ControlCenterInner;
 use crate::control_center::bool;
 use crate::control_center::bool_ui;
 use crate::control_center::combo_box;
+use crate::control_center::combo_box_ui;
 use crate::control_center::drag_value;
 use crate::control_center::grid;
 use crate::control_center::grid_label_ui;
@@ -134,6 +135,24 @@ impl LookAndFeelPane {
                 self.state.split_reuses_container.get(),
                 |v| self.state.set_split_reuses_container(v),
             );
+            combo_box_ui(
+                ui,
+                "Flatten Tree",
+                |ui| {
+                    tip(ui, |ui| {
+                        ui.label(concat!(
+                            "Containers that are left with only one child are replaced by ",
+                            "that child.",
+                        ));
+                        ui.label(concat!(
+                            "`Always` additionally applies this when splitting the only ",
+                            "window in a container.",
+                        ));
+                    });
+                },
+                self.state.flatten_tree.get(),
+                |v| self.state.set_flatten_tree(v),
+            );
             row(ui, "Font", |ui| {
                 let mut v = self.state.theme.font.get().to_string();
                 if text_edit(ui, &mut v).changed() {

src/state.rs

@@ -179,6 +179,7 @@ use crate::tree::ContainerSplit;
 use crate::tree::Direction;
 use crate::tree::DisplayNode;
 use crate::tree::FindTreeUsecase;
+use crate::tree::FlattenTree;
 use crate::tree::FloatNode;
 use crate::tree::FoundNode;
 use crate::tree::LatchListener;
@@ -394,6 +395,7 @@ pub struct State {
     pub color_management_enabled: Cell<bool>,
     pub color_manager: Rc<ColorManager>,
     pub float_above_fullscreen: Cell<bool>,
+    pub flatten_tree: Cell<FlattenTree>,
     pub icons: Icons,
     pub show_pin_icon: Cell<bool>,
     pub cl_matcher_manager: Rc<ClMatcherManager>,
@@ -2314,6 +2316,11 @@ impl State {
         self.root.update_visible(self);
     }
 
+    pub fn set_flatten_tree(&self, v: FlattenTree) {
+        self.flatten_tree.set(v);
+        self.trigger_cci(CCI_LOOK_AND_FEEL);
+    }
+
     pub fn reset_sizes(self: &Rc<Self>) {
         self.theme.sizes.reset(LiveTL);
         self.spaces_changed();

src/tree.rs

@@ -45,6 +45,7 @@ pub use containing::*;
 pub use display::*;
 pub use float::*;
 use jay_config::Direction as JayDirection;
+use jay_config::FlattenTree as ConfigFlattenTree;
 use jay_config::video::Transform as ConfigTransform;
 use jay_config::window::TileState as ConfigTileState;
 use jay_config::workspace::WorkspaceDisplayOrder as ConfigWorkspaceDisplayOrder;
@@ -275,6 +276,48 @@ impl From<JayDirection> for Direction {
     }
 }
 
+#[derive(Copy, Clone, Debug, Hash, Eq, PartialEq, Default, Linearize)]
+pub enum FlattenTree {
+    #[default]
+    Never,
+    Always,
+    OnRemove,
+}
+
+impl TryFrom<ConfigFlattenTree> for FlattenTree {
+    type Error = ();
+
+    fn try_from(value: ConfigFlattenTree) -> Result<Self, Self::Error> {
+        let v = match value {
+            ConfigFlattenTree::Never => FlattenTree::Never,
+            ConfigFlattenTree::Always => FlattenTree::Always,
+            ConfigFlattenTree::OnRemove => FlattenTree::OnRemove,
+            _ => return Err(()),
+        };
+        Ok(v)
+    }
+}
+
+impl Into<ConfigFlattenTree> for FlattenTree {
+    fn into(self) -> ConfigFlattenTree {
+        match self {
+            FlattenTree::Never => ConfigFlattenTree::Never,
+            FlattenTree::Always => ConfigFlattenTree::Always,
+            FlattenTree::OnRemove => ConfigFlattenTree::OnRemove,
+        }
+    }
+}
+
+impl StaticText for FlattenTree {
+    fn text(&self) -> &'static str {
+        match self {
+            FlattenTree::Never => "Never",
+            FlattenTree::Always => "Always",
+            FlattenTree::OnRemove => "On Remove",
+        }
+    }
+}
+
 pub struct NodeIds {
     next: NumCell<u32>,
 }

src/tree/container.rs

@@ -30,6 +30,7 @@ use crate::tree::ContainingNode;
 use crate::tree::Direction;
 use crate::tree::FindTreeResult;
 use crate::tree::FindTreeUsecase;
+use crate::tree::FlattenTree;
 use crate::tree::FloatNode;
 use crate::tree::FoundNode;
 use crate::tree::Node;
@@ -176,6 +177,17 @@ struct MainAxisRange {
     active: bool,
 }
 
+/// The outcome of an attempt to replace a container by its sole remaining child.
+#[derive(Copy, Clone, Debug, Eq, PartialEq)]
+enum ReplaceWithSoleChild {
+    /// The container was replaced by the child and destroyed.
+    Flattened,
+    /// The parent does not accept the child.
+    Rejected,
+    /// The container has no parent, or is fullscreen.
+    NotApplicable,
+}
+
 #[derive(Default)]
 pub struct ContainerNodeState {
     pub split: Cell<ContainerSplit>,
@@ -371,6 +383,28 @@ impl ContainerNode {
         ref_
     }
 
+    fn try_replace_with_sole_child(
+        self: &Rc<Self>,
+        child: Rc<dyn ToplevelNode>,
+    ) -> ReplaceWithSoleChild {
+        let Some(parent) = self.toplevel_data.parent.get() else {
+            return ReplaceWithSoleChild::NotApplicable;
+        };
+        if self.toplevel_data.is_fullscreen[LiveTL].get() {
+            return ReplaceWithSoleChild::NotApplicable;
+        }
+        if !parent.cnode_accepts_child(&*child) {
+            return ReplaceWithSoleChild::Rejected;
+        }
+        parent.cnode_replace_child(self.deref(), child);
+        self.toplevel_data.parent.take();
+        for cn in self.child_nodes.borrow_mut().drain_values() {
+            self.schedule_unlink_child(cn);
+        }
+        self.tl_destroy();
+        ReplaceWithSoleChild::Flattened
+    }
+
     pub fn prepend_child(self: &Rc<Self>, new: Rc<dyn ToplevelNode>) {
         if let Some(child) = self.children.first() {
             self.add_child_before_(&child, new);
@@ -1365,19 +1399,9 @@ impl ContainerNode {
 
         // CASE 1: This is the only child of the container. Replace the container by the child.
         if ns.num_children.get() == 1 {
-            if let Some(parent) = self.toplevel_data.parent.get()
-                && !self.toplevel_data.is_fullscreen[LiveTL].get()
-            {
-                if parent.cnode_accepts_child(&*child) {
-                    parent.cnode_replace_child(self.deref(), child.clone());
-                    self.toplevel_data.parent.take();
-                    for cn in self.child_nodes.borrow_mut().drain_values() {
-                        self.schedule_unlink_child(cn);
-                    }
-                    self.tl_destroy();
-                } else {
-                    move_to_neighboring_output(child);
-                }
+            match self.try_replace_with_sole_child(child.clone()) {
+                ReplaceWithSoleChild::Flattened | ReplaceWithSoleChild::NotApplicable => {}
+                ReplaceWithSoleChild::Rejected => move_to_neighboring_output(child),
             }
             return;
         }
@@ -2400,6 +2424,17 @@ impl ContainingNode for ContainerNode {
         }
         let node = self.schedule_unlink_child(node);
         let num_children = self.adj_ns_num_children(|value| value - 1);
+        if num_children == 1
+            && matches!(
+                self.state.flatten_tree.get(),
+                FlattenTree::Always | FlattenTree::OnRemove
+            )
+            && let Some(remaining) = self.children.first_valid(LiveTL)
+            && self.try_replace_with_sole_child(remaining.node.clone())
+                == ReplaceWithSoleChild::Flattened
+        {
+            return;
+        }
         if num_children == 0 {
             self.tl_destroy();
             return;

src/tree/toplevel.rs

@@ -38,6 +38,7 @@ use crate::tree::ContainerSplit;
 use crate::tree::ContainerTarget;
 use crate::tree::ContainingNode;
 use crate::tree::Direction;
+use crate::tree::FlattenTree;
 use crate::tree::FloatNode;
 use crate::tree::Node;
 use crate::tree::NodeBase;
@@ -1222,7 +1223,7 @@ pub fn toplevel_create_split(state: &Rc<State>, tl: Rc<dyn ToplevelNode>, axis:
         Some(ws) => ws,
         _ => return,
     };
-    if state.split_reuses_container.get()
+    if (state.split_reuses_container.get() || state.flatten_tree.get() == FlattenTree::Always)
         && let Some(pn) = toplevel_parent_container(&*tl)
         && pn.node_state[LiveTL].num_children.get() == 1
     {