問題の個所の範囲を描画してみる。

lltexteditor_debug.diff

--- a/indra/llui/lltexteditor.cpp
+++ b/indra/llui/lltexteditor.cpp
@@ -2175,6 +2175,26 @@ void LLTextEditor::drawPreeditMarker()
                line_start = next_start;
                cur_line++;
        }
+
+       // for debug
+       LLCoordGL coord;
+       LLRect bounds;
+       getPreeditLocation(-1, &coord, &bounds, NULL);
+       LLRect preedit_rect_screen;
+       LLUI::glRectToScreen(bounds, &preedit_rect_screen);
+       LLRect preedit_rect_local;
+       screenRectToLocal(preedit_rect_screen, &preedit_rect_local);
+       gl_rect_2d(preedit_rect_local, LLColor4(1.0, 0.0, 0.0, 1.0), 0);
+
+       S32 query_screen_x;
+       S32 query_screen_y;
+       LLUI::glPointToScreen(coord.mX, coord.mY, &query_screen_x, &query_screen_y);
+
+       S32 query_x;
+       S32 query_y;
+       screenPointToLocal(query_screen_x, query_screen_y, &query_x, &query_y);
+
+       gl_rect_2d(query_x - 1, query_y + 1, query_x + 1, query_y - 1, LLColor4(0.0, 1.0, 0.0, 1.0));
 }