Bug 1520983 - part 2: Make `IMEInputHandler::InsertTextAsComittingCom…

…position()` use `eContentCommandInsertText` event if there is no composition r=m_kato

Like the other browsers, we should stop dispatching a set of composition
events when `insertText:` is called by Emoji picker, accent character
picker of some Western keyboard layouts.  Then, corresponding
`beforeinput` event becomes cancelable like the other browsers'.

Differential Revision: https://phabricator.services.mozilla.com/D114827

modules/libpref/init/StaticPrefList.yaml

@@ -5562,6 +5562,20 @@
   value: false
   mirror: always
 
+#ifdef XP_MACOSX
+# Whether text input without keyboard press nor IME composition should cause a
+# set of composition events or not.  E.g., when you use Emoji picker on macOS,
+# it inserts an Emoji character directly.  If set to true, `compositionstart`,
+# `compositionupdate` and `compositionend` events will be fired, and the
+# correspnding `beforeinput` events are not cancelable.  Otherwise, if set to
+# false, any user input events are not exposed to web apps but only
+# `beforeinput` event is fired as "insert text" as a cancelable event.
+- name: intl.ime.use_composition_events_for_insert_text
+  type: bool
+  value: false
+  mirror: always
+#endif
+
 #---------------------------------------------------------------------------
 # Prefs starting with "javascript."
 #

widget/cocoa/TextInputHandler.mm

@@ -12,6 +12,7 @@
 #include "mozilla/AutoRestore.h"
 #include "mozilla/MiscEvents.h"
 #include "mozilla/MouseEvents.h"
+#include "mozilla/StaticPrefs_intl.h"
 #include "mozilla/Telemetry.h"
 #include "mozilla/TextEventDispatcher.h"
 #include "mozilla/TextEvents.h"
@@ -3789,13 +3790,30 @@ inline NSRange MakeNSRangeFrom(const Maybe<OffsetAndData<uint32_t>>& aOffsetAndD
   nsCocoaUtils::GetStringForNSString([aAttrString string], str);
 
   if (!IsIMEComposing()) {
+    MOZ_DIAGNOSTIC_ASSERT(!str.IsEmpty());
+
     // If there is no selection and replacement range is specified, set the
     // range as selection.
     if (aReplacementRange && aReplacementRange->location != NSNotFound &&
         !NSEqualRanges(SelectedRange(), *aReplacementRange)) {
       NS_ENSURE_TRUE_VOID(SetSelection(*aReplacementRange));
     }
 
+    if (!StaticPrefs::intl_ime_use_composition_events_for_insert_text()) {
+      // In the default settings, we should not use composition events for
+      // inserting text without key press nor IME composition because the
+      // other browsers do so.   This will cause only a cancelable `beforeinput`
+      // event whose `inputType` is `insertText`.
+      WidgetContentCommandEvent insertTextEvent(true, eContentCommandInsertText, mWidget);
+      insertTextEvent.mString = Some(str);
+      DispatchEvent(insertTextEvent);
+      return;
+    }
+
+    // Otherise, emulate an IME composition.  This is our traditional behavior,
+    // but `beforeinput` events are not cancelable since they should be so for
+    // native IME limitation.  So, this is now seriously imcompatible with the
+    // other browsers.
     if (!DispatchCompositionStartEvent()) {
       MOZ_LOG(gLog, LogLevel::Info,
               ("%p IMEInputHandler::InsertTextAsCommittingComposition, "