Refactor Ctrl-6 fix to be clearer and only trigger under Japanese IME

Only limiting this condition to be under Japanese IME helps prevents
unintended effects on other IMEs as well, as this bug is only known to
happen under Japanese IME with Windows shortcuts.

Also update comments to be clear that most of the time you do want to go
through the interpretKeyEvents: path.

Author: ychin

src/MacVim/MMTextViewHelper.m

@@ -180,14 +180,24 @@ - (void)keyDown:(NSEvent *)event
         // and Ctrl-U (why?), so we cannot handle them at all.
         // As a workaround, we do not call interpretKeyEvents: with Ctrl-O or
         // Ctrl-U when there is no marked text.
-        if ([self hasMarkedText]
-                    || !((modControl && !modCommand && !modOption)
-                            && ([unmod characterAtIndex:0] == 'o' ||
-                                [unmod characterAtIndex:0] == 'u'))) {
-            // HACK! interpretKeyEvents: may call insertText: or
-            // doCommandBySelector:, or it may swallow the key (most likely the
-            // current input method used it).  In the first two cases we have to
-            // manually set the below flag to NO if the key wasn't handled.
+        BOOL isJapaneseIME = [[[NSTextInputContext currentInputContext]
+                               selectedKeyboardInputSource]
+                              hasPrefix:@"com.apple.inputmethod.Kotoeri"];
+        BOOL isJapaneseIMCtrlUO = (isJapaneseIME
+                                   && (modControl && !modCommand && !modOption)
+                                   && ([unmod characterAtIndex:0] == 'o' ||
+                                       [unmod characterAtIndex:0] == 'u'));
+
+        if ([self hasMarkedText] || !isJapaneseIMCtrlUO) {
+            // interpretKeyEvents: may call insertText: or doCommandBySelector:,
+            // or it may swallow the key (most likely the current input method
+            // used it).  In the first two cases we have to manually set the
+            // below flag to NO if the key wasn't handled, which allows us to
+            // manually send the key down event.
+            //
+            // doCommandBySelector: will also perform misc translations such as
+            // Ctrl-6 -> Ctrl-^, so this should not be skipped unless there are
+            // special cases like the Japanese IME Ctrl-U issue.
             interpretKeyEventsSwallowedKey = YES;
             [textView interpretKeyEvents:[NSArray arrayWithObject:event]];
             if (interpretKeyEventsSwallowedKey)