Fix USB terminal backspace handling

examples/companion_radio/main.cpp

@@ -139,6 +139,35 @@ static void clearUsbTerminalLine() {
   usb_terminal_line_len = 0;
 }
 
+static void printUsbTerminalInputEcho() {
+  const char* password = nullptr;
+  size_t visible_len = usb_terminal_line_len;
+  if (mesh::cli::parseTerminalArgumentCommand(
+          usb_terminal_line, "login", password)
+      == mesh::cli::TerminalArgumentCommandMatch::Valid) {
+    visible_len = static_cast<size_t>(password - usb_terminal_line);
+  }
+
+  if (visible_len > 0) {
+    Serial.write(reinterpret_cast<const uint8_t*>(usb_terminal_line),
+                 visible_len);
+  }
+  for (size_t i = visible_len; i < usb_terminal_line_len; i++) {
+    Serial.print('*');
+  }
+}
+
+static void redrawUsbTerminalInput() {
+  // The documented picocom `--imap spchex` converts an echoed BS to "[08]".
+  // It deliberately leaves CR untouched, so redraw the edited line with CR
+  // and printable bytes only. Padding clears a removed tab or wide glyph.
+  Serial.print("\r> ");
+  printUsbTerminalInputEcho();
+  Serial.print("        ");
+  Serial.print("\r> ");
+  printUsbTerminalInputEcho();
+}
+
 static bool isUsbTerminalDataConnected() {
 #if defined(RP2040_PLATFORM)
   return (bool)Serial;
@@ -285,8 +314,9 @@ static void serviceUsbTerminal() {
 
     if (c == '\b' || c == 0x7F) {
       if (usb_terminal_line_len > 0) {
-        usb_terminal_line[--usb_terminal_line_len] = 0;
-        Serial.print("\b \b");
+        usb_terminal_line_len = mesh::cli::eraseLastTerminalInput(
+            usb_terminal_line, usb_terminal_line_len);
+        redrawUsbTerminalInput();
       }
       continue;
     }

src/helpers/CLICommandUtils.h

@@ -118,6 +118,20 @@ inline bool shouldMaskTerminalInput(const char* line) {
       == TerminalArgumentCommandMatch::Valid;
 }
 
+// Remove one complete UTF-8 code point from a terminal input buffer. Invalid
+// trailing bytes are still removed safely, and the result remains terminated.
+inline size_t eraseLastTerminalInput(char* line, size_t line_length) {
+  if (line == nullptr || line_length == 0) return 0;
+
+  size_t new_length = line_length - 1;
+  while (new_length > 0
+         && (static_cast<uint8_t>(line[new_length]) & 0xC0) == 0x80) {
+    new_length--;
+  }
+  line[new_length] = 0;
+  return new_length;
+}
+
 inline TerminalChannelCommandMatch parseTerminalChannelMessage(
     const char* command, TerminalChannelMessage& message) {
   message.selector = nullptr;

test/test_cli_command_utils/test_cli_command_utils.cpp

@@ -198,6 +198,24 @@ TEST(CLICommandUtils, MasksOnlyTerminalLoginPasswordInput) {
   EXPECT_FALSE(mesh::cli::shouldMaskTerminalInput("login-status"));
 }
 
+TEST(CLICommandUtils, BackspaceErasesAsciiAndUtf8Characters) {
+  char ascii[] = "trace path 2 7773";
+  size_t length = strlen(ascii);
+
+  length = mesh::cli::eraseLastTerminalInput(ascii, length);
+  EXPECT_STREQ("trace path 2 777", ascii);
+  EXPECT_EQ(strlen(ascii), length);
+
+  char utf8[] = "channel Public hi 👋";
+  length = strlen(utf8);
+  length = mesh::cli::eraseLastTerminalInput(utf8, length);
+  EXPECT_STREQ("channel Public hi ", utf8);
+  EXPECT_EQ(strlen(utf8), length);
+
+  EXPECT_EQ(0u, mesh::cli::eraseLastTerminalInput(nullptr, 0));
+  EXPECT_EQ(0u, mesh::cli::eraseLastTerminalInput(utf8, 0));
+}
+
 TEST(CLICommandUtils, ParsesStrictDecimalValues) {
   float value = 0.0f;