Region scrolling tests and a fix

@tbung fixed a scrolling bug in #6099.  This patch just
adds some tests and takes care of a couple corner cases
that still remained with region scrolling.

term/src/screen.rs

@@ -649,6 +649,8 @@ impl Screen {
     ) {
         let phys_scroll = self.phys_range(scroll_region);
         let num_rows = num_rows.min(phys_scroll.end - phys_scroll.start);
+        let scrollback_ok = scroll_region.start == 0 && self.allow_scrollback;
+        let insert_at_end = scroll_region.end as usize == self.physical_rows;
 
         debug!(
             "scroll_up {:?} num_rows={} phys_scroll={:?}",
@@ -660,15 +662,15 @@ impl Screen {
         // changed by the scroll operation.  For normal newline at the bottom
         // of the screen based scrolling, the StableRowIndex does not change,
         // so we use the scroll region bounds to gate the invalidation.
-        if scroll_region.start != 0 || scroll_region.end as usize != self.physical_rows {
+        if !scrollback_ok {
             for y in phys_scroll.clone() {
                 self.line_mut(y).update_last_change_seqno(seqno);
             }
         }
 
         // if we're going to remove lines due to lack of scrollback capacity,
         // remember how many so that we can adjust our insertion point later.
-        let lines_removed = if scroll_region.start > 0 {
+        let lines_removed = if !scrollback_ok {
             // No scrollback available for these;
             // Remove the scrolled lines
             num_rows
@@ -708,7 +710,7 @@ impl Screen {
                     line.update_last_change_seqno(seqno);
                     line
                 };
-                if scroll_region.end as usize == self.physical_rows {
+                if insert_at_end {
                     self.lines.push_back(line);
                 } else {
                     self.lines.insert(phys_scroll.end - 1, line);
@@ -724,12 +726,10 @@ impl Screen {
             self.lines.remove(remove_idx);
         }
 
-        if remove_idx == 0 && self.allow_scrollback {
+        if remove_idx == 0 && scrollback_ok {
             self.stable_row_index_offset += lines_removed;
         }
 
-        // It's cheaper to push() than it is insert() at the end
-        let push = scroll_region.end as usize == self.physical_rows;
         for _ in 0..to_add {
             let mut line = if default_blank == blank_attr {
                 Line::new(seqno)
@@ -741,12 +741,19 @@ impl Screen {
                 )
             };
             bidi_mode.apply_to_line(&mut line, seqno);
-            if push {
+            if insert_at_end {
                 self.lines.push_back(line);
             } else {
                 self.lines.insert(phys_scroll.end, line);
             }
         }
+
+        // If we have invalidated the StableRowIndex, mark all subsequent lines as dirty
+        if to_remove > 0 || (to_add > 0 && !insert_at_end) {
+            for y in self.phys_range(&(scroll_region.end..self.physical_rows as VisibleRowIndex)) {
+                self.line_mut(y).update_last_change_seqno(seqno);
+            }
+        }
     }
 
     pub fn erase_scrollback(&mut self) {