feat: support horizontal mouse scrolling in buffers

src/diff.rs

@@ -609,6 +609,35 @@ impl DiffState {
         }
     }
 
+    pub(crate) fn bound_horizontal(&self, focus: Focus, left: usize, width: usize) -> usize {
+        let columns = match focus {
+            Focus::Before => self.max_before,
+            Focus::After => self.max_after,
+            Focus::Files => return 0,
+        };
+        left.min(columns.saturating_add(1).saturating_sub(width.max(1)))
+    }
+
+    pub(crate) fn scroll_horizontal(&mut self, focus: Focus, right: bool, width: usize) {
+        let left = match focus {
+            Focus::Before => self.left_before,
+            Focus::After => self.left_after,
+            Focus::Files => return,
+        };
+        let left = if right {
+            left.saturating_add(3)
+        } else {
+            left.saturating_sub(3)
+        };
+        let left = self.bound_horizontal(focus, left, width);
+        match focus {
+            Focus::Before => self.left_before = left,
+            Focus::After => self.left_after = left,
+            Focus::Files => unreachable!(),
+        }
+        self.follow_caret = false;
+    }
+
     pub(crate) fn first_text_focus(&self) -> Focus {
         if self.pane_layout() == PaneLayout::After {
             Focus::After