terminal: avoid VS15 cursor underflow (#13631)

Handle VS15 width changes when the wide grapheme base is directly under
the cursor.

A zero cursor distance previously underflowed while locating the spacer
tail. Debug builds panicked and ReleaseFast computed an out-of-bounds
cell pointer before updating it.

src/terminal/Terminal.zig

@@ -1314,28 +1314,24 @@ pub fn print(self: *Terminal, c: u21) !void {
                     if (prev.cell.wide != .wide) break :narrow;
                     prev.cell.wide = .narrow;
 
-                    // Remove the wide spacer tail
-                    const cell = self.screens.active.cursorCellLeft(prev.left - 1);
-                    cell.wide = .narrow;
-
-                    // Back track the cursor so that we don't end up with
-                    // an extra space after the character. Since xterm is
-                    // not VS aware, it cannot be used as a reference for
-                    // this behavior; but it does follow the principle of
-                    // least surprise, and also matches the behavior that
-                    // can be observed in Kitty, which is one of the only
-                    // other VS aware terminals.
-                    if (self.screens.active.cursor.x == right_limit - 1) {
-                        // If we're already at the right edge, we stay
-                        // here and set the pending wrap to false since
-                        // when we pend a wrap, we only move our cursor once
-                        // even for wide chars (tests verify).
-                        self.screens.active.cursor.pending_wrap = false;
-                    } else {
-                        // Otherwise, move back.
-                        self.screens.active.cursorLeft(1);
+                    // Remove the wide spacer tail. The previous cell may be
+                    // under the cursor, so locate the tail from the wide base
+                    // rather than by subtracting from the cursor distance.
+                    const prev_x = self.screens.active.cursor.x - prev.left;
+                    if (prev_x < self.cols - 1) {
+                        const cells: [*]Cell = @ptrCast(prev.cell);
+                        cells[1].wide = .narrow;
                     }
 
+                    // Place the cursor one cell after the now-narrow base,
+                    // clamped to the right edge. Usually this moves the cursor
+                    // back from after the old tail, but saved cursor state or
+                    // changed margins can leave it directly on the base.
+                    self.screens.active.cursor.pending_wrap = false;
+                    self.screens.active.cursorHorizontalAbsolute(
+                        @min(prev_x + 1, right_limit - 1),
+                    );
+
                     break :narrow;
                 },
 
@@ -5746,6 +5742,68 @@ test "Terminal: VS15 to make narrow character with pending wrap" {
     }
 }
 
+test "Terminal: VS15 narrows wide cell under cursor with wraparound disabled" {
+    var t = try init(testing.io, testing.allocator, .{ .rows = 5, .cols = 5 });
+    defer t.deinit(testing.allocator);
+
+    t.modes.set(.grapheme_cluster, true);
+    t.modes.set(.wraparound, false);
+
+    // First create a wide cell spanning columns 4 and 5.
+    t.setCursorPos(1, 4);
+    try t.print(0x2614);
+
+    // Make column 4 the right margin and put the cursor on the wide base.
+    // With wraparound disabled, grapheme lookup selects the cell under the
+    // cursor when it has content.
+    t.modes.set(.enable_left_and_right_margin, true);
+    t.setLeftAndRightMargin(1, 4);
+    t.setCursorPos(1, 4);
+    try t.print(0xFE0E);
+
+    try testing.expectEqual(@as(usize, 3), t.screens.active.cursor.x);
+    try testing.expect(!t.screens.active.cursor.pending_wrap);
+    const base = t.screens.active.pages.getCell(.{ .screen = .{ .x = 3, .y = 0 } }).?.cell;
+    try testing.expectEqual(Cell.Wide.narrow, base.wide);
+    try testing.expect(base.hasGrapheme());
+    const tail = t.screens.active.pages.getCell(.{ .screen = .{ .x = 4, .y = 0 } }).?.cell;
+    try testing.expectEqual(Cell.Wide.narrow, tail.wide);
+}
+
+test "Terminal: VS15 narrows wide cell under restored pending cursor" {
+    var t = try init(testing.io, testing.allocator, .{ .rows = 5, .cols = 5 });
+    defer t.deinit(testing.allocator);
+
+    t.modes.set(.grapheme_cluster, true);
+    t.modes.set(.enable_left_and_right_margin, true);
+    t.setLeftAndRightMargin(1, 4);
+
+    // Save a pending-wrap cursor at column 4.
+    t.setCursorPos(1, 4);
+    try t.print('X');
+    try testing.expect(t.screens.active.cursor.pending_wrap);
+    t.saveCursor();
+
+    // Widen the margin and replace that cell with a wide character.
+    t.setLeftAndRightMargin(1, 5);
+    t.setCursorPos(1, 4);
+    try t.print(0x2614);
+
+    // Restoring also restores pending_wrap, so grapheme lookup selects the
+    // wide base under the cursor rather than its spacer tail.
+    t.restoreCursor();
+    try testing.expect(t.screens.active.cursor.pending_wrap);
+    try t.print(0xFE0E);
+
+    try testing.expectEqual(@as(usize, 4), t.screens.active.cursor.x);
+    try testing.expect(!t.screens.active.cursor.pending_wrap);
+    const base = t.screens.active.pages.getCell(.{ .screen = .{ .x = 3, .y = 0 } }).?.cell;
+    try testing.expectEqual(Cell.Wide.narrow, base.wide);
+    try testing.expect(base.hasGrapheme());
+    const tail = t.screens.active.pages.getCell(.{ .screen = .{ .x = 4, .y = 0 } }).?.cell;
+    try testing.expectEqual(Cell.Wide.narrow, tail.wide);
+}
+
 test "Terminal: VS16 to make wide character on next line" {
     var t = try init(testing.io, testing.allocator, .{ .rows = 5, .cols = 3 });
     defer t.deinit(testing.allocator);