Add MOZC_IBUS_CANDIDATE_WINDOW environment variable.

If this variable is set to `ibus`, the default Ibus candidate window is used.
If this variable is set to `mozc`, the Mozc candidate window is used.

* #637

PiperOrigin-RevId: 495258411

docs/configurations.md

@@ -174,3 +174,16 @@ engines {
   composition_mode: DIRECT
 }
 ```
+
+
+### Use the default Ibus candidate window
+
+If the environment variable `MOZC_IBUS_CANDIDATE_WINDOW` is set to `ibus`,
+The default Ibus candidate window is used instead of the Mozc candidate window.
+
+If `MOZC_IBUS_CANDIDATE_WINDOW` is set to `mozc`, the Mozc candidate window is
+always used.
+
+Note, the default Ibus candidate window may not have the full features
+we provide to the Mozc candidate window such as information list
+(e.g. word usage dictionary).

src/unix/ibus/mozc_engine.cc

@@ -236,6 +236,15 @@ bool UseMozcCandidateWindow() {
     return false;
   }
 
+  // Follows the user's preference set in the environment variable.
+  const std::string candidate_window = GetEnv("MOZC_IBUS_CANDIDATE_WINDOW");
+  if (candidate_window == "ibus") {
+    return false;
+  }
+  if (candidate_window == "mozc") {
+    return true;
+  }
+
 #ifndef ENABLE_QT_RENDERER
   if (GetEnv("XDG_SESSION_TYPE") == "wayland") {
     // mozc_renderer is not supported on wayland session.