diff --git a/src/ui/chat.rs b/src/ui/chat.rs
index 353ef57..73acb30 100644
--- a/src/ui/chat.rs
+++ b/src/ui/chat.rs
@@ -1,3 +1,4 @@
+mod blocks;
 mod tree;
 
 use std::sync::Arc;
diff --git a/src/ui/chat/blocks.rs b/src/ui/chat/blocks.rs
new file mode 100644
index 0000000..ebe872d
--- /dev/null
+++ b/src/ui/chat/blocks.rs
@@ -0,0 +1,112 @@
+use std::collections::{vec_deque, VecDeque};
+
+use toss::frame::Frame;
+
+use crate::macros::some_or_return;
+use crate::ui::widgets::BoxedWidget;
+
+pub struct Block<I> {
+    id: I,
+    top_line: i32,
+    height: i32,
+    widget: BoxedWidget,
+}
+
+impl<I> Block<I> {
+    pub fn new<W: Into<BoxedWidget>>(frame: &mut Frame, width: u16, id: I, widget: W) -> Self {
+        // Interestingly, rust-analyzer fails to deduce the type of `widget`
+        // here but rustc knows it's a `BoxedWidget`.
+        let widget = widget.into();
+        let size = widget.size(frame, Some(width), None);
+        Self {
+            id,
+            top_line: 0,
+            height: size.height.into(),
+            widget,
+        }
+    }
+}
+
+pub struct Blocks<I> {
+    pub blocks: VecDeque<Block<I>>,
+    /// The top line of the first block. Useful for prepending blocks,
+    /// especially to empty [`Blocks`]s.
+    pub top_line: i32,
+    /// The bottom line of the last block. Useful for appending blocks,
+    /// especially to empty [`Blocks`]s.
+    pub bottom_line: i32,
+}
+
+impl<I> Blocks<I> {
+    pub fn new() -> Self {
+        Self::new_below(0)
+    }
+
+    /// Create a new [`Blocks`] such that the first prepended line will be on
+    /// `line`.
+    pub fn new_below(line: i32) -> Self {
+        Self {
+            blocks: VecDeque::new(),
+            top_line: line + 1,
+            bottom_line: line,
+        }
+    }
+
+    pub fn offset(&mut self, delta: i32) {
+        self.top_line += delta;
+        self.bottom_line += delta;
+        for block in &mut self.blocks {
+            block.top_line += delta;
+        }
+    }
+
+    pub fn push_front(&mut self, mut block: Block<I>) {
+        self.top_line -= block.height;
+        block.top_line = self.top_line;
+        self.blocks.push_front(block);
+    }
+
+    pub fn push_back(&mut self, mut block: Block<I>) {
+        block.top_line = self.bottom_line + 1;
+        self.bottom_line += block.height;
+        self.blocks.push_back(block);
+    }
+
+    pub fn prepend(&mut self, mut layout: Self) {
+        while let Some(block) = layout.blocks.pop_back() {
+            self.push_front(block);
+        }
+    }
+
+    pub fn append(&mut self, mut layout: Self) {
+        while let Some(block) = layout.blocks.pop_front() {
+            self.push_back(block);
+        }
+    }
+}
+
+impl<I: Eq> Blocks<I> {
+    pub fn recalculate_offsets(&mut self, id: I, top_line: i32) {
+        let idx = some_or_return!(self
+            .blocks
+            .iter()
+            .enumerate()
+            .find(|(_, b)| b.id == id)
+            .map(|(i, _)| i));
+
+        self.blocks[idx].top_line = top_line;
+
+        // Propagate changes to top
+        for i in (0..idx).rev() {
+            self.blocks[i].top_line = self.blocks[i + 1].top_line - self.blocks[i].height;
+        }
+        self.top_line = self.blocks.front().expect("blocks nonempty").top_line;
+
+        // Propagate changes to bottom
+        for i in (idx + 1)..self.blocks.len() {
+            self.blocks[i].top_line = self.blocks[i - 1].top_line + self.blocks[i - 1].height;
+        }
+        let bottom = self.blocks.back().expect("blocks nonempty");
+        self.bottom_line = bottom.top_line + bottom.height - 1;
+    }
+}