terminal: rows created from IND should inherit the current bg attr

Any row created from scrolling via IND ("\x1BD") should have it's
background set as the current background. This can be verified in any
terminal with

  $ echo -e "\x1B[41m" && cat -v"

Followed by pressing enter to scroll the screen. We expect to see red
rows appear. Add test case to verify.

Fixes: alacritty/vim_large_screen_scroll

src/terminal/Screen.zig

@@ -1753,10 +1753,31 @@ fn scrollDelta(self: *Screen, delta: isize, viewport_only: bool) !void {
     }
 
     // Ensure we have "written" our last row so that it shows up
-    _ = self.storage.getPtrSlice(
+    const slices = self.storage.getPtrSlice(
         (rows_written_final - 1) * (self.cols + 1),
         self.cols + 1,
     );
+    // We should never be wrapped here
+    assert(slices[1].len == 0);
+
+    // We only grabbed our new row(s), copy cells into the whole slice
+    const dst = slices[0];
+    // The pen we'll use for new cells (only the BG attribute is applied to new
+    // cells)
+    const pen: Cell = if (!self.cursor.pen.attrs.has_bg) .{} else .{
+        .bg = self.cursor.pen.bg,
+        .attrs = .{ .has_bg = true },
+    };
+    @memset(dst, .{ .cell = pen });
+
+    // Then we make sure our row headers are zeroed out. We set
+    // the value to a dirty row header so that the renderer re-draws.
+    var i: usize = 0;
+    while (i < dst.len) : (i += self.cols + 1) {
+        dst[i] = .{ .header = .{
+            .flags = .{ .dirty = true },
+        } };
+    }
 
     if (start_viewport_bottom) {
         // If our viewport is on the bottom, we always update the viewport
@@ -3126,6 +3147,8 @@ test "Screen: scrolling" {
 
     var s = try init(alloc, 3, 5, 0);
     defer s.deinit();
+    s.cursor.pen.bg = .{ .r = 155 };
+    s.cursor.pen.attrs.has_bg = true;
     try s.testWriteString("1ABCD\n2EFGH\n3IJKL");
     try testing.expect(s.viewportIsBottom());
 
@@ -3139,6 +3162,11 @@ test "Screen: scrolling" {
         defer alloc.free(contents);
         try testing.expectEqualStrings("2EFGH\n3IJKL", contents);
     }
+    {
+        // Test that our new row has the correct background
+        const cell = s.getCell(.active, 2, 0);
+        try testing.expectEqual(@as(u8, 155), cell.bg.r);
+    }
 
     // Scrolling to the bottom does nothing
     try s.scroll(.{ .bottom = {} });