screen: when expanding cols, broadcast empty styled cells

src/terminal/Screen.zig

@@ -1894,8 +1894,19 @@ pub fn resize(self: *Screen, rows: usize, cols: usize) !void {
                 new_cursor = .{ .y = self.rowsWritten() - 1, .x = cursor_pos.x };
             }
 
-            // If no reflow, just keep going
+            // If no reflow, just keep going.
             if (!old_row.header().flags.wrap) {
+                // If we have no reflow, we attempt to extend any stylized
+                // cells at the end of the line if there is one.
+                const len = old_row.lenCells();
+                const end = new_row.getCell(len - 1);
+                if ((end.char == 0 or end.char == ' ') and !end.empty()) {
+                    for (len..self.cols) |x| {
+                        const cell = new_row.getCellPtr(x);
+                        cell.* = end;
+                    }
+                }
+
                 y += 1;
                 continue;
             }
@@ -4050,6 +4061,59 @@ test "Screen: resize more cols no reflow" {
     }
 }
 
+test "Screen: resize more cols trailing background colors" {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var s = try init(alloc, 3, 5, 0);
+    defer s.deinit();
+    const str = "1AB";
+    try s.testWriteString(str);
+    const cursor = s.cursor;
+
+    // Color our cells red
+    const pen: Cell = .{ .bg = .{ .r = 0xFF }, .attrs = .{ .has_bg = true } };
+    for (s.cursor.x..s.cols) |x| {
+        const row = s.getRow(.{ .active = s.cursor.y });
+        const cell = row.getCellPtr(x);
+        cell.* = pen;
+    }
+    for ((s.cursor.y + 1)..s.rows) |y| {
+        const row = s.getRow(.{ .active = y });
+        row.fill(pen);
+    }
+
+    try s.resize(3, 10);
+
+    // Cursor should not move
+    try testing.expectEqual(cursor, s.cursor);
+
+    {
+        var contents = try s.testString(alloc, .viewport);
+        defer alloc.free(contents);
+        try testing.expectEqualStrings(str, contents);
+    }
+    {
+        var contents = try s.testString(alloc, .screen);
+        defer alloc.free(contents);
+        try testing.expectEqualStrings(str, contents);
+    }
+
+    // Verify all our trailing cells have the color
+    for (s.cursor.x..s.cols) |x| {
+        const row = s.getRow(.{ .active = s.cursor.y });
+        const cell = row.getCellPtr(x);
+        try testing.expectEqual(pen, cell.*);
+    }
+    for ((s.cursor.y + 1)..s.rows) |y| {
+        const row = s.getRow(.{ .active = y });
+        for (0..s.cols) |x| {
+            const cell = row.getCellPtr(x);
+            try testing.expectEqual(pen, cell.*);
+        }
+    }
+}
+
 test "Screen: resize more cols grapheme map" {
     const testing = std.testing;
     const alloc = testing.allocator;