Move cursor so it is visible when scrolling

src/ui/chat/blocks.rs

@@ -1,4 +1,4 @@
-use std::collections::VecDeque;
+use std::collections::{vec_deque, VecDeque};
 use std::ops::Range;
 
 use toss::frame::Frame;
@@ -66,6 +66,10 @@ impl<I> Blocks<I> {
         }
     }
 
+    pub fn iter(&self) -> vec_deque::Iter<Block<I>> {
+        self.blocks.iter()
+    }
+
     pub fn offset(&mut self, delta: i32) {
         self.top_line += delta;
         self.bottom_line += delta;

src/ui/chat/tree.rs

@@ -60,9 +60,7 @@ impl<M: Msg, S: MsgStore<M>> InnerTreeViewState<M, S> {
             KeyCode::Char('j') | KeyCode::Down => self.move_cursor_down().await,
             KeyCode::Char('g') | KeyCode::Home => self.move_cursor_to_top().await,
             KeyCode::Char('G') | KeyCode::End => self.move_cursor_to_bottom().await,
-            KeyCode::Char('y') if event.modifiers == KeyModifiers::CONTROL => {
-                self.scroll_up(1).await
-            }
+            KeyCode::Char('y') if event.modifiers == KeyModifiers::CONTROL => self.scroll_up(1),
             KeyCode::Char('e') if event.modifiers == KeyModifiers::CONTROL => self.scroll_down(1),
             _ => return false,
         }

src/ui/chat/tree/cursor.rs

@@ -224,17 +224,8 @@ impl<M: Msg, S: MsgStore<M>> InnerTreeViewState<M, S> {
         self.make_cursor_visible = true;
     }
 
-    pub async fn scroll_up(&mut self, amount: i32) {
+    pub fn scroll_up(&mut self, amount: i32) {
         self.scroll += amount;
-
-        if let Cursor::Bottom = self.cursor {
-            // Move cursor to bottommost message, if it exists
-            if let Some(mut id) = self.store.last_tree_id().await {
-                let tree = self.store.tree(&id).await;
-                while Self::find_last_child(&tree, &mut id) {}
-                self.cursor = Cursor::Msg(id);
-            }
-        }
     }
 
     pub fn scroll_down(&mut self, amount: i32) {

src/ui/chat/tree/layout.rs

@@ -308,6 +308,70 @@ impl<M: Msg, S: MsgStore<M>> InnerTreeViewState<M, S> {
         }
     }
 
+    /// Try to obtain a [`Cursor::Msg`] pointing to the block.
+    fn msg_id(block: &Block<BlockId<M::Id>>) -> Option<M::Id> {
+        match &block.id {
+            BlockId::Msg(id) => Some(id.clone()),
+            _ => None,
+        }
+    }
+
+    fn visible(block: &Block<BlockId<M::Id>>, height: i32) -> bool {
+        (1 - block.height..height).contains(&block.top_line)
+    }
+
+    fn move_cursor_so_it_is_visible(
+        &mut self,
+        frame: &mut Frame,
+        blocks: &TreeBlocks<M::Id>,
+    ) -> Option<M::Id> {
+        if !matches!(self.cursor, Cursor::Bottom | Cursor::Msg(_)) {
+            // In all other cases, there is no need to make the cursor visible
+            // since scrolling behaves differently enough.
+            return None;
+        }
+
+        let height = frame.size().height as i32;
+
+        let new_cursor = if matches!(self.cursor, Cursor::Bottom) {
+            blocks
+                .blocks()
+                .iter()
+                .rev()
+                .filter(|b| Self::visible(b, height))
+                .find_map(Self::msg_id)
+        } else {
+            let block = blocks
+                .blocks()
+                .find(&BlockId::from_cursor(&self.cursor))
+                .expect("no cursor found");
+
+            if Self::visible(block, height) {
+                return None;
+            } else if block.top_line < 0 {
+                blocks
+                    .blocks()
+                    .iter()
+                    .filter(|b| Self::visible(b, height))
+                    .find_map(Self::msg_id)
+            } else {
+                blocks
+                    .blocks()
+                    .iter()
+                    .rev()
+                    .filter(|b| Self::visible(b, height))
+                    .find_map(Self::msg_id)
+            }
+        };
+
+        if let Some(id) = new_cursor {
+            self.cursor = Cursor::Msg(id.clone());
+            Some(id)
+        } else {
+            None
+        }
+    }
+
     pub async fn relayout(&mut self, frame: &mut Frame) -> TreeBlocks<M::Id> {
         // The basic idea is this:
         //
@@ -350,9 +414,22 @@ impl<M: Msg, S: MsgStore<M>> InnerTreeViewState<M, S> {
             self.fill_screen_and_clamp_scrolling(frame, &mut blocks)
                 .await;
         } else {
-            // self.move_cursor_so_it_is_visible(&mut blocks); // TODO
-            self.fill_screen_and_clamp_scrolling(frame, &mut blocks)
-                .await;
+            let new_cursor_msg_id = self.move_cursor_so_it_is_visible(frame, &blocks);
+            if let Some(cursor_msg_id) = new_cursor_msg_id {
+                // Moving the cursor invalidates our current blocks, so we sadly
+                // have to either perform an expensive operation or redraw the
+                // entire thing. I'm choosing the latter for now.
+
+                self.last_cursor = self.cursor.clone();
+                self.last_cursor_line = self.cursor_line(&blocks);
+                self.make_cursor_visible = false;
+                self.scroll = 0;
+
+                let last_cursor_path = self.store.path(&cursor_msg_id).await;
+                blocks = self.layout_last_cursor_seed(frame, &last_cursor_path).await;
+                self.fill_screen_and_clamp_scrolling(frame, &mut blocks)
+                    .await;
+            }
         }
 
         self.last_cursor = self.cursor.clone();