term: fix DCH removing cells instead of setting to current bg color

refs: #789

docs/changelog.md

@@ -28,6 +28,7 @@ As features stabilize some brief notes about them will accumulate here.
 * Fixed: ALT + Arrow, PageUp/PageDown, Ins, Del, Home, End incorrectly sent ESC prefixed key sequences. [#892](https://github.com/wez/wezterm/issues/892)
 * New: Added [SendKey](config/lua/keyassignment/SendKey.md) key assignment action that makes it more convenient to rebind the key input that is sent to a pane.
 * Fixed: Crash due to Out of Memory condition when the iTerm2 protocol was used to send excessively large PNG files [#1031](https://github.com/wez/wezterm/issues/1031)
+* Fixed: `DCH` sequence would remove cells and replace them with default-blank cells instead of blank-cells-with-current-bg-color. [#789](https://github.com/wez/wezterm/issues/789)
 
 ### 20210814-124438-54e29167
 

term/src/screen.rs

@@ -319,10 +319,11 @@ impl Screen {
         y: VisibleRowIndex,
         right_margin: usize,
         seqno: SequenceNo,
+        blank_attr: CellAttributes,
     ) {
         let line_idx = self.phys_row(y);
         let line = self.line_mut(line_idx);
-        line.erase_cell_with_margin(x, right_margin, seqno);
+        line.erase_cell_with_margin(x, right_margin, seqno, blank_attr);
     }
 
     /// Set a cell.  the x and y coordinates are relative to the visible screeen

term/src/terminalstate/mod.rs

@@ -1708,9 +1708,10 @@ impl TerminalState {
                     let right_margin = self.left_and_right_margins.end;
                     let limit = (x + n as usize).min(right_margin);
 
+                    let blank_attr = self.pen.clone_sgr_only();
                     let screen = self.screen_mut();
                     for _ in x..limit as usize {
-                        screen.erase_cell(x, y, right_margin, seqno);
+                        screen.erase_cell(x, y, right_margin, seqno, blank_attr.clone());
                     }
                 }
             }

term/src/test/csi.rs

@@ -1,4 +1,32 @@
 use super::*;
+use termwiz::color::AnsiColor;
+
+/// In this issue, the `CSI 2 P` sequence incorrectly removed two
+/// cells from the line, leaving them effectively blank, when those
+/// two cells should have been erased to the current background
+/// color as set by `CSI 40 m`
+#[test]
+fn test_789() {
+    let mut term = TestTerm::new(1, 8, 0);
+    term.print("\x1b[40m\x1b[Kfoo\x1b[2P");
+
+    let black = CellAttributes::default()
+        .set_background(AnsiColor::Black)
+        .clone();
+    let mut line = Line::from_text("foo", &black);
+    line.resize(8, 0);
+    for x in 3..8 {
+        line.set_cell(x, Cell::blank_with_attrs(black.clone()), 0);
+    }
+
+    assert_lines_equal(
+        file!(),
+        line!(),
+        &term.screen().visible_lines(),
+        &[line],
+        Compare::TEXT | Compare::ATTRS,
+    );
+}
 
 #[test]
 fn test_vpa() {
@@ -76,14 +104,14 @@ fn test_dch() {
     term.print("hello world");
     term.cup(1, 0);
     term.print("\x1b[P");
-    assert_visible_contents(&term, file!(), line!(), &["hllo world "]);
+    assert_visible_contents(&term, file!(), line!(), &["hllo world  "]);
 
     term.cup(4, 0);
     term.print("\x1b[2P");
-    assert_visible_contents(&term, file!(), line!(), &["hlloorld "]);
+    assert_visible_contents(&term, file!(), line!(), &["hlloorld    "]);
 
     term.print("\x1b[-2P");
-    assert_visible_contents(&term, file!(), line!(), &["hlloorld "]);
+    assert_visible_contents(&term, file!(), line!(), &["hlloorld    "]);
 }
 
 #[test]

termwiz/src/surface/line.rs

@@ -539,14 +539,23 @@ impl Line {
         self.update_last_change_seqno(seqno);
     }
 
-    pub fn erase_cell_with_margin(&mut self, x: usize, right_margin: usize, seqno: SequenceNo) {
+    pub fn erase_cell_with_margin(
+        &mut self,
+        x: usize,
+        right_margin: usize,
+        seqno: SequenceNo,
+        blank_attr: CellAttributes,
+    ) {
         self.invalidate_implicit_hyperlinks(seqno);
         if x < self.cells.len() {
             self.invalidate_grapheme_at_or_before(x);
             self.cells.remove(x);
         }
-        if right_margin <= self.cells.len() {
-            self.cells.insert(right_margin - 1, Cell::default());
+        if right_margin <= self.cells.len() + 1
+        /* we just removed one */
+        {
+            self.cells
+                .insert(right_margin - 1, Cell::blank_with_attrs(blank_attr));
         }
         self.update_last_change_seqno(seqno);
     }