vim: Fix ctrl-u/ctrl-d (#3044)

- vim: Fix ctrl-d/ctrl-u to match vim (when :set scrolloff=3)

crates/editor/src/editor_tests.rs

@@ -1429,7 +1429,7 @@ async fn test_scroll_page_up_page_down(cx: &mut gpui::TestAppContext) {
         assert_eq!(editor.snapshot(cx).scroll_position(), vec2f(0., 3.));
 
         editor.scroll_screen(&ScrollAmount::Page(-0.5), cx);
-        assert_eq!(editor.snapshot(cx).scroll_position(), vec2f(0., 2.));
+        assert_eq!(editor.snapshot(cx).scroll_position(), vec2f(0., 1.));
         editor.scroll_screen(&ScrollAmount::Page(0.5), cx);
         assert_eq!(editor.snapshot(cx).scroll_position(), vec2f(0., 3.));
     });

crates/editor/src/scroll/scroll_amount.rs

@@ -15,9 +15,13 @@ impl ScrollAmount {
             Self::Line(count) => *count,
             Self::Page(count) => editor
                 .visible_line_count()
-                // subtract one to leave an anchor line
-                // round towards zero (so page-up and page-down are symmetric)
-                .map(|l| (l * count).trunc() - count.signum())
+                .map(|mut l| {
+                    // for full pages subtract one to leave an anchor line
+                    if count.abs() == 1.0 {
+                        l -= 1.0
+                    }
+                    (l * count).trunc()
+                })
                 .unwrap_or(0.),
         }
     }