Getting back to where we started... (Buggy Resizing)

2 files changed, 208 insertions, 501 deletions

diff --git a/zellij-server/src/ui/layout.rs b/zellij-server/src/ui/layout.rs
index a9af665fa..7a19d889a 100644
--- a/zellij-server/src/ui/layout.rs
+++ b/zellij-server/src/ui/layout.rs
@@ -19,17 +19,14 @@ fn split_space_to_parts_vertically(
 
     // First fit in the parameterized sizes
     for size in sizes {
-        let (columns, max_columns) = match size {
+        let columns = match size {
             Some(SplitSize::Percent(percent)) => {
-                ((max_width as f32 * (percent as f32 / 100.0)) as usize, None)
+                (max_width as f32 * (percent as f32 / 100.0)) as usize
             } // TODO: round properly
-            Some(SplitSize::Fixed(size)) => (size as usize, Some(size as usize)),
+            Some(SplitSize::Fixed(size)) => size as usize,
             None => {
                 parts_to_grow.push(current_x_position);
-                (
-                    1, // This is grown later on
-                    None,
-                )
+                1 // This is grown later on
             }
         };
         split_parts.push(PositionAndSize {
@@ -37,7 +34,6 @@ fn split_space_to_parts_vertically(
             y: space_to_split.y,
             columns,
             rows: space_to_split.rows,
-            max_columns,
             ..Default::default()
         });
         current_width += columns;
@@ -87,18 +83,14 @@ fn split_space_to_parts_horizontally(
     let mut parts_to_grow = Vec::new();
 
     for size in sizes {
-        let (rows, max_rows) = match size {
-            Some(SplitSize::Percent(percent)) => (
-                (max_height as f32 * (percent as f32 / 100.0)) as usize,
-                None,
-            ), // TODO: round properly
-            Some(SplitSize::Fixed(size)) => (size as usize, Some(size as usize)),
+        let rows = match size {
+            Some(SplitSize::Percent(percent)) => {
+                (max_height as f32 * (percent as f32 / 100.0)) as usize
+            } // TODO: round properly
+            Some(SplitSize::Fixed(size)) => size as usize,
             None => {
                 parts_to_grow.push(current_y_position);
-                (
-                    1, // This is grown later on
-                    None,
-                )
+                1 // This is grown later on
             }
         };
         split_parts.push(PositionAndSize {
@@ -106,7 +98,6 @@ fn split_space_to_parts_horizontally(
             y: current_y_position,
             columns: space_to_split.columns,
             rows,
-            max_rows,
             ..Default::default()
         });
         current_height += rows;
diff --git a/zellij-server/src/ui/pane_resizer.rs b/zellij-server/src/ui/pane_resizer.rs
index b2001945e..73673a9c5 100644
--- a/zellij-server/src/ui/pane_resizer.rs
+++ b/zellij-server/src/ui/pane_resizer.rs
@@ -1,531 +1,247 @@
 use crate::{os_input_output::ServerOsApi, panes::PaneId, tab::Pane};
+use cassowary::{
+    strength::{REQUIRED, STRONG},
+    Constraint, Solver, Variable,
+    WeightedRelation::*,
+};
 use std::{
-    cmp::Ordering,
     collections::{BTreeMap, HashSet},
+    ops::Not,
 };
 use zellij_utils::pane_size::PositionAndSize;
 
-pub(crate) struct PaneResizer<'a> {
+const GAP_SIZE: usize = 1; // Panes are separated by this number of rows / columns
+
+pub struct PaneResizer<'a> {
     panes: &'a mut BTreeMap<PaneId, Box<dyn Pane>>,
+    vars: BTreeMap<PaneId, (Variable, Variable)>,
+    solver: Solver,
     os_api: &'a mut Box<dyn ServerOsApi>,
 }
 
+#[derive(Debug, Clone, Copy)]
+enum Direction {
+    Horizontal,
+    Vertical,
+}
+
+impl Not for Direction {
+    type Output = Self;
+
+    fn not(self) -> Self::Output {
+        match self {
+            Direction::Horizontal => Direction::Vertical,
+            Direction::Vertical => Direction::Horizontal,
+        }
+    }
+}
+
+#[derive(Debug, Clone, Copy)]
+struct Span {
+    pid: PaneId,
+    direction: Direction,
+    fixed: bool,
+    pos: usize,
+    size: usize,
+    pos_var: Variable,
+    size_var: Variable,
+}
+
 // TODO: currently there are some functions here duplicated with Tab
 // all resizing functions should move here
 
+// FIXME:
+// 1. Rounding causes a loss of ratios, I need to store an internal f64 for
+//    each pane as well as the displayed usize and add custom rounding logic.
+// 2. Vertical resizing doesn't seem to respect the space consumed by the tab
+//    and status bars?
+// 3. A 2x2 layout and simultaneous vertical + horizontal resizing sometimes
+//    leads to unsolvable constraints? Maybe related to 2 (and possibly 1).
+//    I should sanity-check the `spans_in_boundary()` here!
+
 impl<'a> PaneResizer<'a> {
     pub fn new(
         panes: &'a mut BTreeMap<PaneId, Box<dyn Pane>>,
         os_api: &'a mut Box<dyn ServerOsApi>,
     ) -> Self {
-        PaneResizer { panes, os_api }
+        let mut vars = BTreeMap::new();
+        for &k in panes.keys() {
+            vars.insert(k, (Variable::new(), Variable::new()));
+        }
+        PaneResizer {
+            panes,
+            vars,
+            solver: Solver::new(),
+            os_api,
+        }
     }
+
     pub fn resize(
         &mut self,
-        mut current_size: PositionAndSize,
+        current_size: PositionAndSize,
         new_size: PositionAndSize,
     ) -> Option<(isize, isize)> {
-        // (column_difference, row_difference)
-        let mut successfully_resized = false;
-        let mut column_difference: isize = 0;
-        let mut row_difference: isize = 0;
-        match new_size.columns.cmp(&current_size.columns) {
-            Ordering::Greater => {
-                let increase_by = new_size.columns - current_size.columns;
-                if let Some(panes_to_resize) = find_increasable_vertical_chain(
-                    &self.panes,
-                    increase_by,
-                    current_size.columns,
-                    current_size.rows,
-                ) {
-                    self.increase_panes_right_and_push_adjacents_right(
-                        panes_to_resize,
-                        increase_by,
-                    );
-                    column_difference = new_size.columns as isize - current_size.columns as isize;
-                    current_size.columns =
-                        (current_size.columns as isize + column_difference) as usize;
-                    successfully_resized = true;
-                };
-            }
-            Ordering::Less => {
-                let reduce_by = current_size.columns - new_size.columns;
-                if let Some(panes_to_resize) = find_reducible_vertical_chain(
-                    &self.panes,
-                    reduce_by,
-                    current_size.columns,
-                    current_size.rows,
-                ) {
-                    self.reduce_panes_left_and_pull_adjacents_left(panes_to_resize, reduce_by);
-                    column_difference = new_size.columns as isize - current_size.columns as isize;
-                    current_size.columns =
-                        (current_size.columns as isize + column_difference) as usize;
-                    successfully_resized = true;
-                };
-            }
-            Ordering::Equal => (),
-        }
-        match new_size.rows.cmp(&current_size.rows) {
-            Ordering::Greater => {
-                let increase_by = new_size.rows - current_size.rows;
-                if let Some(panes_to_resize) = find_increasable_horizontal_chain(
-                    &self.panes,
-                    increase_by,
-                    current_size.columns,
-                    current_size.rows,
-                ) {
-                    self.increase_panes_down_and_push_down_adjacents(panes_to_resize, increase_by);
-                    row_difference = new_size.rows as isize - current_size.rows as isize;
-                    current_size.rows = (current_size.rows as isize + row_difference) as usize;
-                    successfully_resized = true;
-                };
-            }
-            Ordering::Less => {
-                let reduce_by = current_size.rows - new_size.rows;
-                if let Some(panes_to_resize) = find_reducible_horizontal_chain(
-                    &self.panes,
-                    reduce_by,
-                    current_size.columns,
-                    current_size.rows,
-                ) {
-                    self.reduce_panes_up_and_pull_adjacents_up(panes_to_resize, reduce_by);
-                    row_difference = new_size.rows as isize - current_size.rows as isize;
-                    current_size.rows = (current_size.rows as isize + row_difference) as usize;
-                    successfully_resized = true;
-                };
-            }
-            Ordering::Equal => (),
-        }
-        if successfully_resized {
-            Some((column_difference, row_difference))
-        } else {
-            None
-        }
-    }
-    fn reduce_panes_left_and_pull_adjacents_left(
-        &mut self,
-        panes_to_reduce: Vec<PaneId>,
-        reduce_by: usize,
-    ) {
-        let mut pulled_panes: HashSet<PaneId> = HashSet::new();
-        for pane_id in panes_to_reduce {
-            let (pane_x, pane_y, pane_columns, pane_rows) = {
-                let pane = self.panes.get(&pane_id).unwrap();
-                (pane.x(), pane.y(), pane.columns(), pane.rows())
-            };
-            let panes_to_pull = self.panes.values_mut().filter(|p| {
-                p.x() > pane_x + pane_columns
-                    && (p.y() <= pane_y && p.y() + p.rows() >= pane_y
-                        || p.y() >= pane_y && p.y() + p.rows() <= pane_y + pane_rows)
-            });
-            for pane in panes_to_pull {
-                if !pulled_panes.contains(&pane.pid()) {
-                    pane.pull_left(reduce_by);
-                    pulled_panes.insert(pane.pid());
-                }
-            }
-            self.reduce_pane_width_left(&pane_id, reduce_by);
-        }
-    }
-    fn reduce_panes_up_and_pull_adjacents_up(
-        &mut self,
-        panes_to_reduce: Vec<PaneId>,
-        reduce_by: usize,
-    ) {
-        let mut pulled_panes: HashSet<PaneId> = HashSet::new();
-        for pane_id in panes_to_reduce {
-            let (pane_x, pane_y, pane_columns, pane_rows) = {
-                let pane = self.panes.get(&pane_id).unwrap();
-                (pane.x(), pane.y(), pane.columns(), pane.rows())
-            };
-            let panes_to_pull = self.panes.values_mut().filter(|p| {
-                p.y() > pane_y + pane_rows
-                    && (p.x() <= pane_x && p.x() + p.columns() >= pane_x
-                        || p.x() >= pane_x && p.x() + p.columns() <= pane_x + pane_columns)
-            });
-            for pane in panes_to_pull {
-                if !pulled_panes.contains(&pane.pid()) {
-                    pane.pull_up(reduce_by);
-                    pulled_panes.insert(pane.pid());
-                }
-            }
-            self.reduce_pane_height_up(&pane_id, reduce_by);
-        }
-    }
-    fn increase_panes_down_and_push_down_adjacents(
-        &mut self,
-        panes_to_increase: Vec<PaneId>,
-        increase_by: usize,
-    ) {
-        let mut pushed_panes: HashSet<PaneId> = HashSet::new();
-        for pane_id in panes_to_increase {
-            let (pane_x, pane_y, pane_columns, pane_rows) = {
-                let pane = self.panes.get(&pane_id).unwrap();
-                (pane.x(), pane.y(), pane.columns(), pane.rows())
-            };
-            let panes_to_push = self.panes.values_mut().filter(|p| {
-                p.y() > pane_y + pane_rows
-                    && (p.x() <= pane_x && p.x() + p.columns() >= pane_x
-                        || p.x() >= pane_x && p.x() + p.columns() <= pane_x + pane_columns)
-            });
-            for pane in panes_to_push {
-                if !pushed_panes.contains(&pane.pid()) {
-                    pane.push_down(increase_by);
-                    pushed_panes.insert(pane.pid());
-                }
-            }
-            self.increase_pane_height_down(&pane_id, increase_by);
-        }
-    }
-    fn increase_panes_right_and_push_adjacents_right(
-        &mut self,
-        panes_to_increase: Vec<PaneId>,
-        increase_by: usize,
-    ) {
-        let mut pushed_panes: HashSet<PaneId> = HashSet::new();
-        for pane_id in panes_to_increase {
-            let (pane_x, pane_y, pane_columns, pane_rows) = {
-                let pane = self.panes.get(&pane_id).unwrap();
-                (pane.x(), pane.y(), pane.columns(), pane.rows())
-            };
-            let panes_to_push = self.panes.values_mut().filter(|p| {
-                p.x() > pane_x + pane_columns
-                    && (p.y() <= pane_y && p.y() + p.rows() >= pane_y
-                        || p.y() >= pane_y && p.y() + p.rows() <= pane_y + pane_rows)
-            });
-            for pane in panes_to_push {
-                if !pushed_panes.contains(&pane.pid()) {
-                    pane.push_right(increase_by);
-                    pushed_panes.insert(pane.pid());
-                }
-            }
-            self.increase_pane_width_right(&pane_id, increase_by);
-        }
-    }
-    fn reduce_pane_height_up(&mut self, id: &PaneId, count: usize) {
-        let pane = self.panes.get_mut(id).unwrap();
-        pane.reduce_height_up(count);
-        if let PaneId::Terminal(pid) = id {
-            self.os_api
-                .set_terminal_size_using_fd(*pid, pane.columns() as u16, pane.rows() as u16);
-        }
-    }
-    fn increase_pane_height_down(&mut self, id: &PaneId, count: usize) {
-        let pane = self.panes.get_mut(id).unwrap();
-        pane.increase_height_down(count);
-        if let PaneId::Terminal(pid) = pane.pid() {
-            self.os_api
-                .set_terminal_size_using_fd(pid, pane.columns() as u16, pane.rows() as u16);
-        }
-    }
-    fn increase_pane_width_right(&mut self, id: &PaneId, count: usize) {
-        let pane = self.panes.get_mut(id).unwrap();
-        pane.increase_width_right(count);
-        if let PaneId::Terminal(pid) = pane.pid() {
-            self.os_api
-                .set_terminal_size_using_fd(pid, pane.columns() as u16, pane.rows() as u16);
-        }
-    }
-    fn reduce_pane_width_left(&mut self, id: &PaneId, count: usize) {
-        let pane = self.panes.get_mut(id).unwrap();
-        pane.reduce_width_left(count);
-        if let PaneId::Terminal(pid) = pane.pid() {
-            self.os_api
-                .set_terminal_size_using_fd(pid, pane.columns() as u16, pane.rows() as u16);
-        }
+        let col_delta = new_size.columns as isize - current_size.columns as isize;
+        let row_delta = new_size.rows as isize - current_size.rows as isize;
+        if col_delta != 0 {
+            let spans = self.solve_direction(Direction::Horizontal, new_size.columns)?;
+            self.collapse_spans(&spans);
+        }
+        self.solver.reset();
+        if row_delta != 0 {
+            let spans = self.solve_direction(Direction::Vertical, new_size.rows)?;
+            self.collapse_spans(&spans);
+        }
+        Some((col_delta, row_delta))
     }
-}
 
-fn find_next_increasable_horizontal_pane(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    right_of: &dyn Pane,
-    increase_by: usize,
-) -> Option<PaneId> {
-    let next_pane_candidates = panes.values().filter(
-        |p| {
-            p.x() == right_of.x() + right_of.columns() + 1 && p.horizontally_overlaps_with(right_of)
-        }, // TODO: the name here is wrong, it should be vertically_overlaps_with
-    );
-    let resizable_candidates =
-        next_pane_candidates.filter(|p| p.can_increase_height_by(increase_by));
-    resizable_candidates.fold(None, |next_pane_id, p| match next_pane_id {
-        Some(next_pane) => {
-            let next_pane = panes.get(&next_pane).unwrap();
-            if next_pane.y() < p.y() {
-                next_pane_id
-            } else {
-                Some(p.pid())
-            }
+    fn solve_direction(&mut self, direction: Direction, space: usize) -> Option<Vec<Span>> {
+        let mut grid = Vec::new();
+        for boundary in self.grid_boundaries(direction) {
+            grid.push(self.spans_in_boundary(direction, boundary));
         }
-        None => Some(p.pid()),
-    })
-}
 
-fn find_next_increasable_vertical_pane(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    below: &dyn Pane,
-    increase_by: usize,
-) -> Option<PaneId> {
-    let next_pane_candidates = panes.values().filter(
-        |p| p.y() == below.y() + below.rows() + 1 && p.vertically_overlaps_with(below), // TODO: the name here is wrong, it should be horizontally_overlaps_with
-    );
-    let resizable_candidates =
-        next_pane_candidates.filter(|p| p.can_increase_width_by(increase_by));
-    resizable_candidates.fold(None, |next_pane_id, p| match next_pane_id {
-        Some(next_pane) => {
-            let next_pane = panes.get(&next_pane).unwrap();
-            if next_pane.x() < p.x() {
-                next_pane_id
-            } else {
-                Some(p.pid())
-            }
-        }
-        None => Some(p.pid()),
-    })
-}
+        let constraints: Vec<_> = grid
+            .iter()
+            .flat_map(|s| constrain_spans(space, s))
+            .collect();
 
-fn find_next_reducible_vertical_pane(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    below: &dyn Pane,
-    reduce_by: usize,
-) -> Option<PaneId> {
-    let next_pane_candidates = panes.values().filter(
-        |p| p.y() == below.y() + below.rows() + 1 && p.vertically_overlaps_with(below), // TODO: the name here is wrong, it should be horizontally_overlaps_with
-    );
-    let resizable_candidates = next_pane_candidates.filter(|p| p.can_reduce_width_by(reduce_by));
-    resizable_candidates.fold(None, |next_pane_id, p| match next_pane_id {
-        Some(next_pane) => {
-            let next_pane = panes.get(&next_pane).unwrap();
-            if next_pane.x() < p.x() {
-                next_pane_id
-            } else {
-                Some(p.pid())
-            }
-        }
-        None => Some(p.pid()),
-    })
-}
+        // FIXME: This line needs to be restored before merging!
+        //self.solver.add_constraints(&constraints).ok()?;
+        self.solver.add_constraints(&constraints).unwrap();
+        Some(grid.into_iter().flatten().collect())
+    }
 
-fn find_next_reducible_horizontal_pane(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    right_of: &dyn Pane,
-    reduce_by: usize,
-) -> Option<PaneId> {
-    let next_pane_candidates = panes.values().filter(
-        |p| {
-            p.x() == right_of.x() + right_of.columns() + 1 && p.horizontally_overlaps_with(right_of)
-        }, // TODO: the name here is wrong, it should be vertically_overlaps_with
-    );
-    let resizable_candidates = next_pane_candidates.filter(|p| p.can_reduce_height_by(reduce_by));
-    resizable_candidates.fold(None, |next_pane_id, p| match next_pane_id {
-        Some(next_pane) => {
-            let next_pane = panes.get(&next_pane).unwrap();
-            if next_pane.y() < p.y() {
-                next_pane_id
+    fn grid_boundaries(&self, direction: Direction) -> Vec<(usize, usize)> {
+        // Select the spans running *perpendicular* to the direction of resize
+        let spans: Vec<Span> = self
+            .panes
+            .values()
+            .map(|p| self.get_span(!direction, p.as_ref()))
+            .collect();
+
+        let mut last_edge = 0;
+        let mut bounds = Vec::new();
+        loop {
+            let mut spans_on_edge: Vec<&Span> =
+                spans.iter().filter(|p| p.pos == last_edge).collect();
+            spans_on_edge.sort_unstable_by_key(|s| s.size);
+            if let Some(next) = spans_on_edge.first() {
+                let next_edge = last_edge + next.size;
+                bounds.push((last_edge, next_edge));
+                last_edge = next_edge + GAP_SIZE;
             } else {
-                Some(p.pid())
+                break;
             }
         }
-        None => Some(p.pid()),
-    })
-}
-
-fn find_increasable_horizontal_chain(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    increase_by: usize,
-    screen_width: usize,
-    screen_height: usize, // TODO: this is the previous size (make this clearer)
-) -> Option<Vec<PaneId>> {
-    let mut horizontal_coordinate = 0;
-    loop {
-        if horizontal_coordinate == screen_height {
-            return None;
-        }
+        bounds
+    }
 
-        match panes
+    fn spans_in_boundary(&self, direction: Direction, boundary: (usize, usize)) -> Vec<Span> {
+        let (start, end) = boundary;
+        let bwn = |v| start <= v && v < end;
+        let mut spans: Vec<_> = self
+            .panes
             .values()
-            .find(|p| p.x() == 0 && p.y() == horizontal_coordinate)
-        {
-            Some(leftmost_pane) => {
-                if !leftmost_pane.can_increase_height_by(increase_by) {
-                    horizontal_coordinate = leftmost_pane.y() + leftmost_pane.rows() + 1;
-                    continue;
-                }
-                let mut panes_to_resize = vec![];
-                let mut current_pane = leftmost_pane;
-                loop {
-                    panes_to_resize.push(current_pane.pid());
-                    if current_pane.x() + current_pane.columns() == screen_width {
-                        return Some(panes_to_resize);
-                    }
-                    match find_next_increasable_horizontal_pane(
-                        panes,
-                        current_pane.as_ref(),
-                        increase_by,
-                    ) {
-                        Some(next_pane_id) => {
-                            current_pane = panes.get(&next_pane_id).unwrap();
-                        }
-                        None => {
-                            horizontal_coordinate = leftmost_pane.y() + leftmost_pane.rows() + 1;
-                            break;
-                        }
-                    };
-                }
-            }
-            None => {
-                return None;
-            }
-        }
+            .filter(|p| {
+                let s = self.get_span(!direction, p.as_ref());
+                bwn(s.pos) || bwn(s.pos + s.size)
+            })
+            .map(|p| self.get_span(direction, p.as_ref()))
+            .collect();
+        spans.sort_unstable_by_key(|s| s.pos);
+        spans
     }
-}
 
-fn find_increasable_vertical_chain(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    increase_by: usize,
-    screen_width: usize,
-    screen_height: usize, // TODO: this is the previous size (make this clearer)
-) -> Option<Vec<PaneId>> {
-    let mut vertical_coordinate = 0;
-    loop {
-        if vertical_coordinate == screen_width {
-            return None;
+    fn get_span(&self, direction: Direction, pane: &dyn Pane) -> Span {
+        let pas = pane.position_and_size();
+        let (pos_var, size_var) = self.vars[&pane.pid()];
+        match direction {
+            Direction::Horizontal => Span {
+                pid: pane.pid(),
+                direction,
+                fixed: pas.cols_fixed,
+                pos: pas.x,
+                size: pas.columns,
+                pos_var,
+                size_var,
+            },
+            Direction::Vertical => Span {
+                pid: pane.pid(),
+                direction,
+                fixed: pas.rows_fixed,
+                pos: pas.y,
+                size: pas.rows,
+                pos_var,
+                size_var,
+            },
         }
+    }
 
-        match panes
-            .values()
-            .find(|p| p.y() == 0 && p.x() == vertical_coordinate)
-        {
-            Some(topmost_pane) => {
-                if !topmost_pane.can_increase_width_by(increase_by) {
-                    vertical_coordinate = topmost_pane.x() + topmost_pane.columns() + 1;
-                    continue;
-                }
-                let mut panes_to_resize = vec![];
-                let mut current_pane = topmost_pane;
-                loop {
-                    panes_to_resize.push(current_pane.pid());
-                    if current_pane.y() + current_pane.rows() == screen_height {
-                        return Some(panes_to_resize);
-                    }
-                    match find_next_increasable_vertical_pane(
-                        panes,
-                        current_pane.as_ref(),
-                        increase_by,
-                    ) {
-                        Some(next_pane_id) => {
-                            current_pane = panes.get(&next_pane_id).unwrap();
-                        }
-                        None => {
-                            vertical_coordinate = topmost_pane.x() + topmost_pane.columns() + 1;
-                            break;
-                        }
-                    };
-                }
-            }
-            None => {
-                return None;
+    fn collapse_spans(&mut self, spans: &[Span]) {
+        for span in spans {
+            let solver = &self.solver; // Hand-holding the borrow-checker
+            let pane = self.panes.get_mut(&span.pid).unwrap();
+            let fetch_usize = |v| solver.get_value(v).round() as usize;
+            match span.direction {
+                Direction::Horizontal => pane.change_pos_and_size(&PositionAndSize {
+                    x: fetch_usize(span.pos_var),
+                    columns: fetch_usize(span.size_var),
+                    ..pane.position_and_size()
+                }),
+                Direction::Vertical => pane.change_pos_and_size(&PositionAndSize {
+                    y: fetch_usize(span.pos_var),
+                    rows: fetch_usize(span.size_var),
+                    ..pane.position_and_size()
+                }),
+            }
+            if let PaneId::Terminal(pid) = pane.pid() {
+                self.os_api.set_terminal_size_using_fd(
+                    pid,
+                    pane.columns() as u16,
+                    pane.rows() as u16,
+                );
             }
         }
     }
 }
 
-fn find_reducible_horizontal_chain(
-    panes: &BTreeMap<PaneId, Box<dyn Pane>>,
-    reduce_by: usize,
-    screen_width: usize,
-    screen_height: usize, // TODO: this is the previous size (make this clearer)
-) -> Option<Vec<PaneId>> {
-    let mut horizontal_coordinate = 0;
-    loop {
-        if horizontal_coordinate == screen_height {
-            return None;
-        }
+fn constrain_spans(space: usize, spans: &[Span]) -> HashSet<Constraint> {
+    let mut constraints = HashSet::new();
 
-        match panes
-            .values()
-            .find(|p| p.x() == 0 && p.y() == horizontal_coordinate)
-