terminal: cloneFrom clears destination

src/terminal/page.zig

@@ -220,9 +220,6 @@ pub const Page = struct {
     /// If the other page has more columns, the extra columns will be
     /// truncated. If the other page has fewer columns, the extra columns
     /// will be zeroed.
-    ///
-    /// The current page is assumed to be empty. We will not clear any
-    /// existing data in the current page.
     pub fn cloneFrom(
         self: *Page,
         other: *const Page,
@@ -232,11 +229,6 @@ pub const Page = struct {
         assert(y_start <= y_end);
         assert(y_end <= other.size.rows);
         assert(y_end - y_start <= self.size.rows);
-        if (comptime std.debug.runtime_safety) {
-            // The current page must be empty.
-            assert(self.styles.count(self.memory) == 0);
-            assert(self.graphemeCount() == 0);
-        }
 
         const other_rows = other.rows.ptr(other.memory)[y_start..y_end];
         const rows = self.rows.ptr(self.memory)[0 .. y_end - y_start];
@@ -254,15 +246,23 @@ pub const Page = struct {
         dst_row: *Row,
         src_row: *const Row,
     ) !void {
+        const cell_len = @min(self.size.cols, other.size.cols);
+        const other_cells = src_row.cells.ptr(other.memory)[0..cell_len];
+        const cells = dst_row.cells.ptr(self.memory)[0..cell_len];
+
+        // If our destination has styles or graphemes then we need to
+        // clear some state.
+        if (dst_row.grapheme or dst_row.styled) {
+            self.clearCells(dst_row, 0, cells.len);
+            assert(!dst_row.grapheme);
+            assert(!dst_row.styled);
+        }
+
         // Copy all the row metadata but keep our cells offset
         const cells_offset = dst_row.cells;
         dst_row.* = src_row.*;
         dst_row.cells = cells_offset;
 
-        const cell_len = @min(self.size.cols, other.size.cols);
-        const other_cells = src_row.cells.ptr(other.memory)[0..cell_len];
-        const cells = dst_row.cells.ptr(self.memory)[0..cell_len];
-
         // If we have no managed memory in the row, we can just copy.
         if (!dst_row.grapheme and !dst_row.styled) {
             fastmem.copy(Cell, cells, other_cells);
@@ -1278,3 +1278,47 @@ test "Page cloneFrom graphemes" {
         try testing.expectEqual(@as(u21, 0), rac.cell.content.codepoint);
     }
 }
+
+test "Page cloneFrom frees dst graphemes" {
+    var page = try Page.init(.{
+        .cols = 10,
+        .rows = 10,
+        .styles = 8,
+    });
+    defer page.deinit();
+    for (0..page.capacity.rows) |y| {
+        const rac = page.getRowAndCell(1, y);
+        rac.cell.* = .{
+            .content_tag = .codepoint,
+            .content = .{ .codepoint = @intCast(y + 1) },
+        };
+    }
+
+    // Clone
+    var page2 = try Page.init(.{
+        .cols = 10,
+        .rows = 10,
+        .styles = 8,
+    });
+    defer page2.deinit();
+    for (0..page2.capacity.rows) |y| {
+        const rac = page2.getRowAndCell(1, y);
+        rac.cell.* = .{
+            .content_tag = .codepoint,
+            .content = .{ .codepoint = @intCast(y + 1) },
+        };
+        try page2.appendGrapheme(rac.row, rac.cell, 0x0A);
+    }
+
+    // Clone from page which has no graphemes.
+    try page2.cloneFrom(&page, 0, page.size.rows);
+
+    // Read it again
+    for (0..page2.capacity.rows) |y| {
+        const rac = page2.getRowAndCell(1, y);
+        try testing.expectEqual(@as(u21, @intCast(y + 1)), rac.cell.content.codepoint);
+        try testing.expect(!rac.row.grapheme);
+        try testing.expect(!rac.cell.hasGrapheme());
+    }
+    try testing.expectEqual(@as(usize, 0), page2.graphemeCount());
+}