fix(compatibility): full screen scroll region (#915)

* fix(compatibility): properly handle scrolling when scroll region is full screen

* style(fmt): make clippy and rustfmt happy

4 files changed, 169 insertions, 6 deletions

diff --git a/src/tests/fixtures/scroll_region_full_screen b/src/tests/fixtures/scroll_region_full_screen
new file mode 100755
index 000000000..c4ee3bf27
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+++ b/src/tests/fixtures/scroll_region_full_screen
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diff --git a/zellij-server/src/panes/grid.rs b/zellij-server/src/panes/grid.rs
index 706e0e549..c4796e8a3 100644
--- a/zellij-server/src/panes/grid.rs
+++ b/zellij-server/src/panes/grid.rs
@@ -374,6 +374,10 @@ pub struct Grid {
     alternative_lines_above_viewport_and_cursor: Option<(VecDeque<Row>, Vec<Row>, Cursor)>,
     cursor: Cursor,
     saved_cursor_position: Option<Cursor>,
+    // FIXME: change scroll_region to be (usize, usize) - where the top line is always the first
+    // line of the viewport and the bottom line the last unless it's changed with CSI r and friends
+    // Having it as an Option causes lots of complexity and issues, and according to DECSTBM, this
+    // should be the behaviour anyway
     scroll_region: Option<(usize, usize)>,
     active_charset: CharsetIndex,
     preceding_char: Option<TerminalCharacter>,
@@ -936,13 +940,30 @@ impl Grid {
                     // the state is corrupted
                     return;
                 }
-                self.viewport.remove(scroll_region_top);
-                let columns = VecDeque::from(vec![EMPTY_TERMINAL_CHARACTER; self.width]);
-                if self.viewport.len() >= scroll_region_bottom {
-                    self.viewport
-                        .insert(scroll_region_bottom, Row::from_columns(columns).canonical());
-                } else {
+                if scroll_region_bottom == self.height - 1 && scroll_region_top == 0 {
+                    let row_count_to_transfer = 1;
+                    let transferred_rows_count = transfer_rows_from_viewport_to_lines_above(
+                        &mut self.viewport,
+                        &mut self.lines_above,
+                        row_count_to_transfer,
+                        self.width,
+                    );
+                    self.scrollback_buffer_lines = subtract_isize_from_usize(
+                        self.scrollback_buffer_lines,
+                        transferred_rows_count,
+                    );
+                    let columns = VecDeque::from(vec![EMPTY_TERMINAL_CHARACTER; self.width]);
                     self.viewport.push(Row::from_columns(columns).canonical());
+                    self.selection.move_up(1);
+                } else {
+                    self.viewport.remove(scroll_region_top);
+                    let columns = VecDeque::from(vec![EMPTY_TERMINAL_CHARACTER; self.width]);
+                    if self.viewport.len() >= scroll_region_bottom {
+                        self.viewport
+                            .insert(scroll_region_bottom, Row::from_columns(columns).canonical());
+                    } else {
+                        self.viewport.push(Row::from_columns(columns).canonical());
+                    }
                 }
                 self.output_buffer.update_all_lines(); // TODO: only update scroll region lines
                 return;
@@ -1226,6 +1247,7 @@ impl Grid {
     pub fn set_scroll_region(&mut self, top_line_index: usize, bottom_line_index: Option<usize>) {
         let bottom_line_index = bottom_line_index.unwrap_or(self.height);
         self.scroll_region = Some((top_line_index, bottom_line_index));
+        self.move_cursor_to(0, 0, EMPTY_TERMINAL_CHARACTER); // DECSTBM moves the cursor to column 1 line 1 of the page
     }
     pub fn clear_scroll_region(&mut self) {
         self.scroll_region = None;
diff --git a/zellij-server/src/panes/unit/grid_tests.rs b/zellij-server/src/panes/unit/grid_tests.rs
index 57ebf405e..d21006049 100644
--- a/zellij-server/src/panes/unit/grid_tests.rs
+++ b/zellij-server/src/panes/unit/grid_tests.rs
@@ -1022,3 +1022,23 @@ pub fn insert_wide_characters_in_existing_line() {
     }
     assert_snapshot!(format!("{:?}", grid));
 }
+
+#[test]
+pub fn full_screen_scroll_region_and_scroll_up() {
+    // this test is a regression test for a bug
+    // where the scroll region would be set to the
+    // full viewport and then scrolling up would cause
+    // lines to get deleted from the viewport rather
+    // than moving to "lines_above"
+    let mut vte_parser = vte::Parser::new();
+    let mut grid = Grid::new(54, 80, Palette::default());
+    let fixture_name = "scroll_region_full_screen";
+    let content = read_fixture(fixture_name);
+    for byte in content {
+        vte_parser.advance(&mut grid, byte);
+    }
+    grid.scroll_up_one_line();
+    grid.scroll_up_one_line();
+    grid.scroll_up_one_line();
+    assert_snapshot!(format!("{:?}", grid));
+}
diff --git a/zellij-server/src/panes/unit/snapshots/zellij_server__panes__grid__grid_tests__full_screen_scroll_region_and_scroll_up.snap b/zellij-server/src/panes/unit/snapshots/zellij_server__panes__grid__grid_tests__full_screen_scroll_region_and_scroll_up.snap
new file mode 100644
index 000000000..2c1509092
--- /dev/null
+++ b/zellij-server/src/panes/unit/snapshots/zellij_server__panes__grid__grid_tests__full_screen_scroll_region_and_scroll_up.snap
@@ -0,0 +1,60 @@
+---
+source: zellij-server/src/panes/./unit/grid_tests.rs
+expression: "format!(\"{:?}\", grid)"
+
+---
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+49 (C): line54                                                                          
+50 (C): line55                                                                          
+51 (C): line56                                                                          
+52 (C): line57                                                                          
+53 (C): line58                                                                          
+