Simplify behaiver of Screen.actual.cursor

For most of Screen::update, Screen.actual.cursor was measured from
the start of the rendered commandline. However, at the end of the function,
because of cursor inability to move the viewport up, `self.r#move(0, 0);`
implicitly changed Screen.actual.cursor to be measured from the minimum
of the viewport and the start of the commandline.

Change this to always measure cursor position from the start of the
commandline.

Author: kerty0

src/screen.rs

@@ -155,7 +155,7 @@ pub struct ScreenData {
     screen_width: Option<usize>,
 
     /// Virtual cursor position used for writing to `line_datas`,
-    /// and also the viewport final cursor position.
+    /// and also final cursor position from the start of rendered commandline.
     cursor: Cursor,
 
     /// Number of prompt lines rendered on the screen.
@@ -494,10 +494,8 @@ impl Screen {
                     mut cursor,
                     scroll_amount,
                 } = scrolled_cursor;
-                if scroll_amount != 0 {
-                    if !is_final_rendering {
-                        self.desired.line_datas = self.desired.line_datas.split_off(scroll_amount);
-                    }
+                if scroll_amount != 0 && !is_final_rendering {
+                    self.desired.line_datas = self.desired.line_datas.split_off(scroll_amount);
                     cursor.y -= scroll_amount;
                 }
                 cursor
@@ -1321,7 +1319,7 @@ impl Screen {
         // Don't do it when running in midnight_commander because of
         // https://midnight-commander.org/ticket/4258.
         if !MIDNIGHT_COMMANDER_HACK.load() {
-            self.r#move(0, 0);
+            self.r#move(0, self.actual.cursor.y);
         }
 
         // Clear remaining lines (if any) if we haven't cleared the screen.