feat: scroll viewport independently of the cursor

src/editor.rs

@@ -633,6 +633,26 @@ core_actions![
         Action::Up,
         &[Mode::Normal, Mode::Insert, Mode::Visual],
     ),
+    (
+        "core.scroll_up",
+        Action::ScrollUp,
+        &[Mode::Normal, Mode::Visual]
+    ),
+    (
+        "core.scroll_down",
+        Action::ScrollDown,
+        &[Mode::Normal, Mode::Visual]
+    ),
+    (
+        "core.scroll_half_up",
+        Action::ScrollHalfUp,
+        &[Mode::Normal, Mode::Visual]
+    ),
+    (
+        "core.scroll_half_down",
+        Action::ScrollHalfDown,
+        &[Mode::Normal, Mode::Visual]
+    ),
     (
         "core.line_start",
         Action::LineStart,
@@ -1322,6 +1342,8 @@ fn default_bindings() -> HashMap<Mode, HashMap<Input, Binding>> {
         for (key, action) in [
             (Input::Ctrl('k'), CloneAbove),
             (Input::Ctrl('e'), CloneBelow),
+            (Input::Ctrl('u'), ScrollHalfUp),
+            (Input::Ctrl('d'), ScrollHalfDown),
             (Input::Escape, Normal),
         ] {
             bindings
@@ -1580,6 +1602,7 @@ struct ViewState {
     command_line: String,
     caret_reveal_revision: u64,
     lsp_center: Cell<Option<(u64, u64, Cursor)>>,
+    viewport_scroll: Cell<(isize, isize)>,
     insert_start: Option<Snapshot>,
     pending_edit: Option<EditRecipe>,
     visual_motions: Option<Vec<Motion>>,
@@ -1600,6 +1623,7 @@ impl ViewState {
             command_line: String::new(),
             caret_reveal_revision: 0,
             lsp_center: Cell::new(None),
+            viewport_scroll: Cell::new((0, 0)),
             insert_start: None,
             pending_edit: None,
             visual_motions: None,
@@ -5787,8 +5811,14 @@ impl Editor {
             ))
     }
 
+    pub(crate) fn take_viewport_scroll(&self, height: usize) -> isize {
+        let (rows, halves) = self.active.view.viewport_scroll.take();
+        rows.saturating_add(halves.saturating_mul((height / 2).max(1) as isize))
+    }
+
     fn reveal_caret(&mut self) {
         self.active.view.lsp_center.set(None);
+        self.active.view.viewport_scroll.set((0, 0));
         self.active.view.caret_reveal_revision =
             self.active.view.caret_reveal_revision.wrapping_add(1);
     }
@@ -7678,6 +7708,7 @@ impl Editor {
         self.refresh_resolver_status();
         self.refresh_resolver_comparison();
         if self.diff_interacting()
+            && self.active.view.viewport_scroll.get() == (0, 0)
             && let Some(diff) = self.interaction_diff_mut()
         {
             diff.follow_caret = true;
@@ -8246,6 +8277,29 @@ impl Editor {
             | Action::LspDiagnostics => unreachable!("catalog Ex actions dispatch above"),
             Action::JumpBack => self.traverse_jumps(false, count),
             Action::JumpForward => self.traverse_jumps(true, count),
+            Action::ScrollUp
+            | Action::ScrollDown
+            | Action::ScrollHalfUp
+            | Action::ScrollHalfDown => {
+                if self.document_actions_available() {
+                    let amount = isize::try_from(count).unwrap_or(isize::MAX);
+                    let amount = if matches!(action, Action::ScrollUp | Action::ScrollHalfUp) {
+                        -amount
+                    } else {
+                        amount
+                    };
+                    let (mut rows, mut halves) = self.active.view.viewport_scroll.get();
+                    if matches!(action, Action::ScrollHalfUp | Action::ScrollHalfDown) {
+                        halves = halves.saturating_add(amount);
+                    } else {
+                        rows = rows.saturating_add(amount);
+                    }
+                    self.active.view.viewport_scroll.set((rows, halves));
+                    if let Some(diff) = self.interaction_diff_mut() {
+                        diff.follow_caret = false;
+                    }
+                }
+            }
             Action::SeekForward | Action::SeekBackward => {
                 self.seek_pending = Some((action == Action::SeekForward, count));
             }