fix(chord_composer): more safely handle the placeholder ZWSP

sometimes Rime outputs an invalid character, invisible among text or being
visualized as diamond with question mark in macOS. I observed this bug often
when I was using combo pinyin but still not sure if this code has to do with
the issue. just mark the placeholder segment "phony" to prevent accidentally
commiting the ZWSP character.

src/rime/gear/chord_composer.cc

@@ -13,8 +13,7 @@
 #include <rime/schema.h>
 #include <rime/gear/chord_composer.h>
 
-// U+FEFF works better with MacType
-static const char* kZeroWidthSpace = "\xef\xbb\xbf";  //"\xe2\x80\x8b";
+static const char* kZeroWidthSpace = "\xe2\x80\x8b";  // U+200B
 
 namespace rime {
 
@@ -118,8 +117,9 @@ ProcessResult ChordComposer::ProcessChordingKey(const KeyEvent& key_event) {
   return kAccepted;
 }
 
-inline static bool is_composing_text(Context* ctx) {
-  return ctx->IsComposing() && ctx->input() != kZeroWidthSpace;
+inline static bool is_composing(Context* ctx) {
+  return !ctx->composition().empty() &&
+      !ctx->composition().back().HasTag("phony");
 }
 
 ProcessResult ChordComposer::ProcessKeyEvent(const KeyEvent& key_event) {
@@ -133,7 +133,7 @@ ProcessResult ChordComposer::ProcessKeyEvent(const KeyEvent& key_event) {
   int ch = key_event.keycode();
   if (!is_key_up && ch >= 0x20 && ch <= 0x7e) {
     // save raw input
-    if (!is_composing_text(engine_->context()) || !raw_sequence_.empty()) {
+    if (!is_composing(engine_->context()) || !raw_sequence_.empty()) {
       raw_sequence_.push_back(ch);
       DLOG(INFO) << "update raw sequence: " << raw_sequence_;
     }
@@ -164,18 +164,17 @@ void ChordComposer::UpdateChord() {
   string code = SerializeChord();
   prompt_format_.Apply(&code);
   if (comp.empty()) {
-    // add an invisbile place holder segment
+    // add a placeholder segment
     // 1. to cheat ctx->IsComposing() == true
     // 2. to attach chord prompt to while chording
-    ctx->PushInput(kZeroWidthSpace);
-    if (comp.empty()) {
-      LOG(ERROR) << "failed to update chord.";
-      return;
-    }
-    comp.back().tags.insert("phony");
-  }
-  comp.back().tags.insert("chord_prompt");
-  comp.back().prompt = code;
+    ctx->set_input(kZeroWidthSpace);
+    Segment placeholder(0, ctx->input().length());
+    placeholder.tags.insert("phony");
+    ctx->composition().AddSegment(placeholder);
+  }
+  auto& last_segment = comp.back();
+  last_segment.tags.insert("chord_prompt");
+  last_segment.prompt = code;
 }
 
 void ChordComposer::FinishChord() {
@@ -207,20 +206,20 @@ void ChordComposer::ClearChord() {
     return;
   Context* ctx = engine_->context();
   Composition& comp = ctx->composition();
-  if (comp.empty()) {
+  if (comp.empty())
     return;
+  auto& last_segment = comp.back();
+  if (comp.size() == 1 && last_segment.HasTag("phony")) {
+    ctx->Clear();
   }
-  if (comp.input().substr(comp.back().start) == kZeroWidthSpace) {
-    ctx->PopInput(ctx->caret_pos() - comp.back().start);
-  }
-  else if (comp.back().HasTag("chord_prompt")) {
-    comp.back().prompt.clear();
-    comp.back().tags.erase("chord_prompt");
+  else if (last_segment.HasTag("chord_prompt")) {
+    last_segment.prompt.clear();
+    last_segment.tags.erase("chord_prompt");
   }
 }
 
 void ChordComposer::OnContextUpdate(Context* ctx) {
-  if (is_composing_text(ctx)) {
+  if (is_composing(ctx)) {
     composing_ = true;
   }
   else if (composing_) {