diff --git a/src/macros.rs b/src/macros.rs
new file mode 100644
index 0000000..d1036dd
--- /dev/null
+++ b/src/macros.rs
@@ -0,0 +1,15 @@
+macro_rules! some_or_return {
+    ($e:expr) => {
+        match $e {
+            None => return,
+            Some(result) => result,
+        }
+    };
+    ($e:expr, $ret:expr) => {
+        match $e {
+            None => return $ret,
+            Some(result) => result,
+        }
+    };
+}
+pub(crate) use some_or_return;
diff --git a/src/main.rs b/src/main.rs
index 5efbed6..22b5113 100644
--- a/src/main.rs
+++ b/src/main.rs
@@ -5,6 +5,7 @@
 mod euph;
 mod export;
 mod logger;
+mod macros;
 mod replies;
 mod store;
 mod ui;
diff --git a/src/ui/chat/tree.rs b/src/ui/chat/tree.rs
index 7525935..9fb3424 100644
--- a/src/ui/chat/tree.rs
+++ b/src/ui/chat/tree.rs
@@ -1,5 +1,5 @@
 // mod action;
-// mod blocks;
+mod blocks;
 // mod cursor;
 // mod layout;
 // mod render;
diff --git a/src/ui/chat/tree/blocks.rs b/src/ui/chat/tree/blocks.rs
index 8be0adb..6b971f5 100644
--- a/src/ui/chat/tree/blocks.rs
+++ b/src/ui/chat/tree/blocks.rs
@@ -1,85 +1,77 @@
-//! Intermediate representation of messages as blocks of lines.
+//! Intermediate representation of chat history as blocks of things.
 
 use std::collections::VecDeque;
 
-use chrono::{DateTime, Utc};
 use toss::styled::Styled;
 
-use super::{util, Cursor};
+use crate::macros::some_or_return;
 
-pub struct Block<I> {
-    pub id: I,
-    pub line: i32,
-    pub height: i32,
-    pub cursor: bool,
-    pub time: Option<DateTime<Utc>>,
-    pub indent: usize,
-    pub body: BlockBody,
+pub enum MarkerBlock<I> {
+    After(I),
+    Bottom,
 }
 
-impl<I> Block<I> {
-    pub fn msg(
-        id: I,
-        indent: usize,
-        time: DateTime<Utc>,
-        nick: Styled,
-        lines: Vec<Styled>,
-    ) -> Self {
-        Self {
-            id,
-            line: 0,
-            height: lines.len() as i32,
-            indent,
-            time: Some(time),
-            cursor: false,
-            body: BlockBody::Msg(MsgBlock { nick, lines }),
-        }
-    }
-
-    pub fn placeholder(id: I, indent: usize) -> Self {
-        Self {
-            id,
-            line: 0,
-            height: 1,
-            indent,
-            time: None,
-            cursor: false,
-            body: BlockBody::Placeholder,
-        }
-    }
-
-    pub fn time(mut self, time: DateTime<Utc>) -> Self {
-        self.time = Some(time);
-        self
-    }
-}
-pub enum BlockBody {
-    Msg(MsgBlock),
+pub enum MsgContent {
+    Msg { nick: Styled, lines: Vec<Styled> },
     Placeholder,
 }
 
-pub struct MsgBlock {
-    pub nick: Styled,
-    pub lines: Vec<Styled>,
+pub struct MsgBlock<I> {
+    id: I,
+    cursor: bool,
+    content: MsgContent,
+}
+
+impl<I> MsgBlock<I> {
+    pub fn height(&self) -> i32 {
+        match &self.content {
+            MsgContent::Msg { lines, .. } => lines.len() as i32,
+            MsgContent::Placeholder => 1,
+        }
+    }
+}
+
+pub struct ComposeBlock {
+    // TODO Editor widget
+}
+
+pub enum BlockBody<I> {
+    Marker(MarkerBlock<I>),
+    Msg(MsgBlock<I>),
+    Compose(ComposeBlock),
+}
+
+pub struct Block<I> {
+    line: i32,
+    indent: usize,
+    body: BlockBody<I>,
+}
+
+impl<I> Block<I> {
+    pub fn height(&self) -> i32 {
+        match &self.body {
+            BlockBody::Marker(m) => 0,
+            BlockBody::Msg(m) => m.height(),
+            BlockBody::Compose(e) => todo!(),
+        }
+    }
 }
 
 /// Pre-layouted messages as a sequence of blocks.
 ///
 /// These blocks are straightforward to render, but also provide a level of
-/// abstraction between the layouting and actual displaying of messages. This
-/// might be useful in the future to ensure the cursor is always on a visible
-/// message, for example.
+/// abstraction between the layouting and actual displaying of messages.
 ///
 /// The following equation describes the relationship between the
 /// [`Blocks::top_line`] and [`Blocks::bottom_line`] fields:
 ///
-/// `bottom_line - top_line + 1 = sum of all heights`
+/// `bottom_line - top_line = sum of all heights - 1`
 ///
 /// This ensures that `top_line` is always the first line and `bottom_line` is
 /// always the last line in a nonempty [`Blocks`]. In an empty layout, the
 /// equation simplifies to
 ///
-/// `top_line = bottom_line + 1`
+/// `bottom_line = top_line - 1`
 pub struct Blocks<I> {
     pub blocks: VecDeque<Block<I>>,
     /// The top line of the first block. Useful for prepending blocks,
@@ -90,66 +82,75 @@ pub struct Blocks<I> {
     pub bottom_line: i32,
 }
 
-impl<I: PartialEq> Blocks<I> {
-    pub fn new() -> Self {
-        Self::new_below(0)
+impl<I> Blocks<I> {
+    pub fn new(initial: Block<I>) -> Self {
+        let top_line = initial.line;
+        let bottom_line = top_line + initial.height() - 1;
+        let mut blocks = VecDeque::new();
+        blocks.push_back(initial);
+        Self {
+            blocks,
+            top_line,
+            bottom_line,
+        }
     }
 
-    /// Create a new [`Blocks`] such that prepending a single line will result
-    /// in `top_line = bottom_line = line`.
-    pub fn new_below(line: i32) -> Self {
+    pub fn new_empty() -> Self {
         Self {
             blocks: VecDeque::new(),
-            top_line: line + 1,
-            bottom_line: line,
+            top_line: 0,
+            bottom_line: -1,
         }
     }
 
-    fn mark_cursor(&mut self, id: &I) -> usize {
-        let mut cursor = None;
-        for (i, block) in self.blocks.iter_mut().enumerate() {
-            if &block.id == id {
-                block.cursor = true;
-                if cursor.is_some() {
-                    panic!("more than one cursor in blocks");
-                }
-                cursor = Some(i);
-            }
-        }
-        cursor.expect("no cursor in blocks")
+    pub fn find<F>(&self, f: F) -> Option<&Block<I>>
+    where
+        F: Fn(&Block<I>) -> bool,
+    {
+        self.blocks.iter().find(|b| f(b))
     }
 
-    pub fn calculate_offsets_with_cursor(&mut self, cursor: &Cursor<I>, height: u16) {
-        let cursor_index = self.mark_cursor(&cursor.id);
-        let cursor_line = util::proportion_to_line(height, cursor.proportion);
+    fn find_index_and_line<F>(&self, f: F) -> Option<(usize, i32)>
+    where
+        F: Fn(&Block<I>) -> Option<i32>,
+    {
+        self.blocks
+            .iter()
+            .enumerate()
+            .find_map(|(i, b)| f(b).map(|l| (i, l)))
+    }
 
-        // Propagate lines from cursor to both ends
-        self.blocks[cursor_index].line = cursor_line;
-        for i in (0..cursor_index).rev() {
-            // let succ_line = self.0[i + 1].line;
-            // let curr = &mut self.0[i];
-            // curr.line = succ_line - curr.height;
-            self.blocks[i].line = self.blocks[i + 1].line - self.blocks[i].height;
+    /// Update the offsets such that the line of the first block with a `Some`
+    /// return value becomes that value.
+    pub fn recalculate_offsets<F>(&mut self, f: F)
+    where
+        F: Fn(&Block<I>) -> Option<i32>,
+    {
+        let (idx, line) = some_or_return!(self.find_index_and_line(f));
+
+        // Propagate lines from index to both ends
+        self.blocks[idx].line = line;
+        for i in (0..idx).rev() {
+            self.blocks[i].line = self.blocks[i + 1].line - self.blocks[i].height();
         }
-        for i in (cursor_index + 1)..self.blocks.len() {
-            // let pred = &self.0[i - 1];
-            // self.0[i].line = pred.line + pred.height;
-            self.blocks[i].line = self.blocks[i - 1].line + self.blocks[i - 1].height;
+        for i in (idx + 1)..self.blocks.len() {
+            self.blocks[i].line = self.blocks[i - 1].line + self.blocks[i - 1].height();
         }
+
         self.top_line = self.blocks.front().expect("blocks nonempty").line;
         let bottom = self.blocks.back().expect("blocks nonempty");
-        self.bottom_line = bottom.line + bottom.height - 1;
+        self.bottom_line = bottom.line + bottom.height() - 1;
     }
 
     pub fn push_front(&mut self, mut block: Block<I>) {
-        self.top_line -= block.height;
+        self.top_line -= block.height();
         block.line = self.top_line;
         self.blocks.push_front(block);
     }
 
     pub fn push_back(&mut self, mut block: Block<I>) {
         block.line = self.bottom_line + 1;
-        self.bottom_line += block.height;
+        self.bottom_line += block.height();
         self.blocks.push_back(block);
     }
 
@@ -172,8 +173,4 @@ impl<I: PartialEq> Blocks<I> {
             block.line += delta;
         }
     }
-
-    pub fn find(&self, id: &I) -> Option<&Block<I>> {
-        self.blocks.iter().find(|b| &b.id == id)
-    }
 }