fix(terminal): move cursor left if VS15 makes cell narrow (#8538)

Without this change, a phantom space appears after any character with
default emoji presentation that is converted to text with VS15. The only
other terminal I know of that respects variation selectors is Kitty, and
it walks the cursor back, which feels like the best choice, since that
way the behavior is observable (no way to know if the terminal supports
variation selectors otherwise without hard-coding that info per term)
and "dumb" programs like `cat` will output things correctly, and not
gain a phantom space after any VS15'd emoji.

> [!NOTE]
> ### Tests should be added for this behavior, including edge cases like
with cursor pending wrap

src/terminal/Terminal.zig

@@ -391,6 +391,24 @@ pub fn print(self: *Terminal, c: u21) !void {
                         const cell = self.screen.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.screen.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.screen.cursor.pending_wrap = false;
+                        } else {
+                            // Otherwise, move back.
+                            self.screen.cursorLeft(1);
+                        }
+
                         break :narrow;
                     },
 
@@ -3348,13 +3366,57 @@ test "Terminal: VS15 to make narrow character" {
     // Enable grapheme clustering
     t.modes.set(.grapheme_cluster, true);
 
-    try t.print(0x26C8); // Thunder cloud and rain
+    try t.print(0x2614); // Umbrella with rain drops, width=2
+    try testing.expect(t.isDirty(.{ .screen = .{ .x = 0, .y = 0 } }));
+    t.clearDirty();
+
+    // We should have 2 cells taken up. It is one character but "wide".
+    try testing.expectEqual(@as(usize, 0), t.screen.cursor.y);
+    try testing.expectEqual(@as(usize, 2), t.screen.cursor.x);
+
+    try t.print(0xFE0E); // VS15 to make narrow
+    try testing.expect(t.isDirty(.{ .screen = .{ .x = 0, .y = 0 } }));
+    t.clearDirty();
+
+    // VS15 should send us back a cell since our char is no longer wide.
+    try testing.expectEqual(@as(usize, 0), t.screen.cursor.y);
+    try testing.expectEqual(@as(usize, 1), t.screen.cursor.x);
+
+    {
+        const str = try t.plainString(testing.allocator);
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("☔︎", str);
+    }
+
+    {
+        const list_cell = t.screen.pages.getCell(.{ .screen = .{ .x = 0, .y = 0 } }).?;
+        const cell = list_cell.cell;
+        try testing.expectEqual(@as(u21, 0x2614), cell.content.codepoint);
+        try testing.expect(cell.hasGrapheme());
+        try testing.expectEqual(Cell.Wide.narrow, cell.wide);
+        const cps = list_cell.node.data.lookupGrapheme(cell).?;
+        try testing.expectEqual(@as(usize, 1), cps.len);
+    }
+}
+
+test "Terminal: VS15 on already narrow emoji" {
+    var t = try init(testing.allocator, .{ .rows = 5, .cols = 5 });
+    defer t.deinit(testing.allocator);
+
+    // Enable grapheme clustering
+    t.modes.set(.grapheme_cluster, true);
+
+    try t.print(0x26C8); // Thunder cloud and rain, width=1
     try testing.expect(t.isDirty(.{ .screen = .{ .x = 0, .y = 0 } }));
     t.clearDirty();
     try t.print(0xFE0E); // VS15 to make narrow
     try testing.expect(t.isDirty(.{ .screen = .{ .x = 0, .y = 0 } }));
     t.clearDirty();
 
+    // Character takes up one cell
+    try testing.expectEqual(@as(usize, 0), t.screen.cursor.y);
+    try testing.expectEqual(@as(usize, 1), t.screen.cursor.x);
+
     {
         const str = try t.plainString(testing.allocator);
         defer testing.allocator.free(str);
@@ -3372,6 +3434,48 @@ test "Terminal: VS15 to make narrow character" {
     }
 }
 
+test "Terminal: VS15 to make narrow character with pending wrap" {
+    var t = try init(testing.allocator, .{ .rows = 5, .cols = 2 });
+    defer t.deinit(testing.allocator);
+
+    // Enable grapheme clustering
+    t.modes.set(.grapheme_cluster, true);
+
+    try t.print(0x2614); // Umbrella with rain drops, width=2
+    try testing.expect(t.isDirty(.{ .screen = .{ .x = 0, .y = 0 } }));
+    t.clearDirty();
+
+    // We only move one because we're in a pending wrap state.
+    try testing.expectEqual(@as(usize, 0), t.screen.cursor.y);
+    try testing.expectEqual(@as(usize, 1), t.screen.cursor.x);
+    try testing.expect(t.screen.cursor.pending_wrap);
+
+    try t.print(0xFE0E); // VS15 to make narrow
+    try testing.expect(t.isDirty(.{ .screen = .{ .x = 0, .y = 0 } }));
+    t.clearDirty();
+
+    // VS15 should clear the pending wrap state
+    try testing.expectEqual(@as(usize, 0), t.screen.cursor.y);
+    try testing.expectEqual(@as(usize, 1), t.screen.cursor.x);
+    try testing.expect(!t.screen.cursor.pending_wrap);
+
+    {
+        const str = try t.plainString(testing.allocator);
+        defer testing.allocator.free(str);
+        try testing.expectEqualStrings("☔︎", str);
+    }
+
+    {
+        const list_cell = t.screen.pages.getCell(.{ .screen = .{ .x = 0, .y = 0 } }).?;
+        const cell = list_cell.cell;
+        try testing.expectEqual(@as(u21, 0x2614), cell.content.codepoint);
+        try testing.expect(cell.hasGrapheme());
+        try testing.expectEqual(Cell.Wide.narrow, cell.wide);
+        const cps = list_cell.node.data.lookupGrapheme(cell).?;
+        try testing.expectEqual(@as(usize, 1), cps.len);
+    }
+}
+
 test "Terminal: VS16 to make wide character with mode 2027" {
     var t = try init(testing.allocator, .{ .rows = 5, .cols = 5 });
     defer t.deinit(testing.allocator);