1 files changed, 61 insertions, 1 deletions
diff --git a/src/tui_engine.zig b/src/tui_engine.zig
index 05b0b89..59bce06 100644
--- a/src/tui_engine.zig
+++ b/src/tui_engine.zig
@@ -610,7 +610,25 @@ pub const Engine = struct {
     /// `total_lines - 1`), not one row below it. `differential` relies on this
     /// to compute how far to move up.
     fn fullRedraw(self: *Engine, frames: anytype, new_total: usize, clear: bool) Error!void {
-        if (clear) try self.writer.writeAll(terminal.seq.full_clear);
+        if (clear) {
+            try self.writer.writeAll(terminal.seq.full_clear);
+        } else {
+            // Non-clearing full reprint (layout change / first paint after
+            // scrolled content). The cursor may be anywhere in the previously
+            // rendered region; move it to `viewport_top` so the reprint
+            // overwrites the correct rows rather than appending below them.
+            // First paint has viewport_top == 0 and hw_cursor_row == 0, so
+            // this is a no-op for the initial frame.
+            const start_row = self.hw_cursor_row;
+            const target = self.viewport_top;
+            if (target < start_row) {
+                try self.cursorUp(start_row - target);
+            } else if (target > start_row) {
+                try self.cursorDown(target - start_row);
+            }
+            try self.writer.writeAll(terminal.seq.carriage_return);
+            try self.writer.writeAll(terminal.seq.clear_to_end);
+        }
         self.cursor_hint = null;
         var global_line: usize = 0;
         var first = true;
@@ -1909,3 +1927,45 @@ test "scheduler: nextDeadline reports remaining window" {
     try testing.expectEqual(@as(?i128, 8 * std.time.ns_per_ms), s.nextDeadline(0));
     try testing.expectEqual(@as(?i128, 0), s.nextDeadline(8 * std.time.ns_per_ms));
 }
+
+test "layout change reprint homes cursor to viewport_top (no duplication when cursor was repositioned)" {
+    // Regression: when the hardware cursor was parked at an input row (above
+    // end-of-content) and a layout change triggered a full reprint, fullRedraw
+    // wrote all lines starting from the CURRENT cursor row rather than
+    // viewport_top, duplicating previously rendered content in the terminal.
+    // The fix: non-clearing fullRedraw moves the cursor to viewport_top first.
+    var buf = std.Io.Writer.Allocating.init(testing.allocator);
+    defer buf.deinit();
+    var eng = makeEngine(&buf, 80, 100); // tall: viewport_top stays 0
+    defer eng.deinit();
+
+    // First frame: 3-line component (welcome + user + input).
+    var body = FakeComponent{
+        .scripts = &.{ &.{ "welcome", "user", "input" }, &.{ "welcome", "user", "input" } },
+        .first_changed = &.{ 0, 0 },
+    };
+    try eng.addComponent(body.comp());
+    try eng.render();
+    // Simulate the app parking the cursor at the input row (row 1, mid-content)
+    // as it would after positioning the hardware cursor for IME anchoring.
+    eng.hw_cursor_row = 1;
+
+    // Layout change: add a new component (thinking block spawn).
+    body.advance();
+    var thinking = FakeComponent{ .scripts = &.{&.{"thinking..."}}, .first_changed = &.{0} };
+    buf.clearRetainingCapacity();
+    _ = eng.removeComponent(body.comp()); // drain
+    try eng.addComponent(body.comp()); // re-add (rebuildEngineList shape)
+    try eng.addComponent(thinking.comp());
+    try eng.render();
+
+    const out = buf.written();
+    // Must NOT wipe scrollback.
+    try testing.expect(std.mem.indexOf(u8, out, terminal.seq.full_clear) == null);
+    // Must reposition the cursor to viewport_top (row 0) from hw_cursor_row (1)
+    // before reprinting: that's one cursorUp(1) = "\x1b[1A" followed by \r.
+    try testing.expect(std.mem.indexOf(u8, out, "\x1b[1A") != null);
+    // Content must be present exactly once.
+    try testing.expectEqual(@as(usize, 1), std.mem.count(u8, out, "welcome"));
+    try testing.expectEqual(@as(usize, 1), std.mem.count(u8, out, "thinking..."));
+}