Bug 1484126 - part 18: Make HTMLEditor::InsertTableCellsWithTransaction() use CellData::NextColumnIndex() instead of |CellData::mCurrent::mColumn + CellData::mColSpan| r=m_kato

Differential Revision: https://phabricator.services.mozilla.com/D8355

--HG--
extra : moz-landing-system : lando

editor/libeditor/HTMLTableEditor.cpp

@@ -217,8 +217,8 @@ HTMLEditor::InsertTableCellsWithTransaction(int32_t aNumberOfCellsToInsert,
       newCellIndex = cellDataAtSelection.mCurrent.mColumn;
       break;
     case InsertPosition::eAfterSelectedCell:
-      newCellIndex =
-        cellDataAtSelection.mCurrent.mColumn + cellDataAtSelection.mColSpan;
+      MOZ_ASSERT(!cellDataAtSelection.IsSpannedFromOtherRowOrColumn());
+      newCellIndex = cellDataAtSelection.NextColumnIndex();
       break;
     default:
       MOZ_ASSERT_UNREACHABLE("Invalid InsertPosition");