apprt/embedded: do not translate control characters

macOS translates inputs such as shift+tab into the control character tab
(ascii 0x09). Linux/GTK does not translate character inputs except to
printable characters. We don't want control character translations
because these are all handled manually by our key encoder (i.e.
translating ctrl+c to 0x03).

src/apprt/embedded.zig

@@ -480,6 +480,13 @@ pub const Surface = struct {
                 // If we aren't composing, then we set our preedit to
                 // empty no matter what.
                 self.core_surface.preeditCallback(null) catch {};
+
+                // If the text is just a single non-printable ASCII character
+                // then we clear the text. We handle non-printables in the
+                // key encoder manual (such as tab, ctrl+c, etc.)
+                if (result.text.len == 1 and result.text[0] < 0x20) {
+                    break :translate .{ .composing = false, .text = "" };
+                }
             }
 
             break :translate result;

src/input/KeyEncoder.zig

@@ -15,6 +15,8 @@ const KittyEntry = @import("kitty.zig").Entry;
 const kitty_entries = @import("kitty.zig").entries;
 const KittyFlags = terminal.kitty.KeyFlags;
 
+const log = std.log.scoped(.key_encoder);
+
 event: key.KeyEvent,
 
 /// The state of various modes of a terminal that impact encoding.
@@ -29,6 +31,8 @@ pub fn encode(
     self: *const KeyEncoder,
     buf: []u8,
 ) ![]const u8 {
+    // log.debug("encode {}", .{self.*});
+
     if (self.kitty_flags.int() != 0) return try self.kitty(buf);
     return try self.legacy(buf);
 }
@@ -863,6 +867,22 @@ test "kitty: up arrow with utf8" {
     try testing.expectEqualStrings("\x1b[A", actual);
 }
 
+test "kitty: shift+tab" {
+    var buf: [128]u8 = undefined;
+    var enc: KeyEncoder = .{
+        .event = .{
+            .key = .tab,
+            .mods = .{ .shift = true },
+            .utf8 = "", // tab
+        },
+
+        .kitty_flags = .{ .disambiguate = true, .report_alternates = true },
+    };
+
+    const actual = try enc.kitty(&buf);
+    try testing.expectEqualStrings("\x1b[9;2u", actual);
+}
+
 test "legacy: ctrl+alt+c" {
     var buf: [128]u8 = undefined;
     var enc: KeyEncoder = .{