ImGui: a bare modifier key does not end the C-k prefix

After C-k, pressing Shift (for C-k C, C-k ^ or a shifted symbol), or
pressing Ctrl again for C-k C-d, reached the k-prefix handler as a key
with no ASCII value, which reported an unknown command and left the
prefix; the suffix was then inserted as text. Modifier keys pressed on
their own are now ignored while the prefix waits.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

ImGuiInputHandler.cc

@@ -157,6 +157,24 @@ map_key(const SDL_Keycode key,
 
 	// If we are in k-prefix, interpret the very next key via the C-k keymap immediately.
 	if (k_prefix) {
+		// A modifier pressed on its own (Shift for C-k C or C-k ^, Ctrl
+		// pressed again for C-k C-d) is not the suffix: keep waiting.
+		switch (key) {
+		case SDLK_LSHIFT:
+		case SDLK_RSHIFT:
+		case SDLK_LCTRL:
+		case SDLK_RCTRL:
+		case SDLK_LALT:
+		case SDLK_RALT:
+		case SDLK_LGUI:
+		case SDLK_RGUI:
+		case SDLK_CAPSLOCK:
+		case SDLK_MODE:
+			out.hasCommand = false;
+			return true;
+		default:
+			break;
+		}
 		esc_meta = false;
 		// Normalize to ASCII; preserve case for letters using Shift
 		int ascii_key = 0;