vim: Fix ctrl-b not moving the cursor (#17808)

Closes #17687

Release Notes:

- Fixed `ctrl-b` not moving the cursor.

---------

Co-authored-by: Abdelhakim Qbaich <abdelhakim@qbaich.com>
Co-authored-by: Pete LeVasseur <plevasseur@gmail.com>

crates/editor/src/scroll/scroll_amount.rs

@@ -1,6 +1,18 @@
 use serde::Deserialize;
 use ui::{px, Pixels};
 
+#[derive(Debug)]
+pub enum ScrollDirection {
+    Upwards,
+    Downwards,
+}
+
+impl ScrollDirection {
+    pub fn is_upwards(&self) -> bool {
+        matches!(self, ScrollDirection::Upwards)
+    }
+}
+
 #[derive(Debug, Clone, PartialEq, Deserialize)]
 pub enum ScrollAmount {
     // Scroll N lines (positive is towards the end of the document)
@@ -15,7 +27,7 @@ impl ScrollAmount {
             Self::Line(count) => *count,
             Self::Page(count) => {
                 // for full pages subtract one to leave an anchor line
-                if count.abs() == 1.0 {
+                if self.is_full_page() {
                     visible_line_count -= 1.0
                 }
                 (visible_line_count * count).trunc()
@@ -29,4 +41,19 @@ impl ScrollAmount {
             ScrollAmount::Page(x) => px(height.0 * x),
         }
     }
+
+    pub fn is_full_page(&self) -> bool {
+        match self {
+            ScrollAmount::Page(count) if count.abs() == 1.0 => true,
+            _ => false,
+        }
+    }
+
+    pub fn direction(&self) -> ScrollDirection {
+        match self {
+            Self::Line(amount) if amount.is_sign_positive() => ScrollDirection::Downwards,
+            Self::Page(amount) if amount.is_sign_positive() => ScrollDirection::Downwards,
+            _ => ScrollDirection::Upwards,
+        }
+    }
 }