Fix horizontally merged cells in RTF to DOCX converter

Author: manfromarce

documentation/Supported_features.MD

@@ -100,7 +100,7 @@ However, some charts may already be OLE objects (Excel/MS Graph object; usually
 - ❌ Paragraph group formatting
 - ❌ Styles (however the formatting is usually written inline in RTF for compatibility reasons, even if there is an associated style too)  
 - 🟡 Lists (note: there are still issues that need to be resolved)
-- ❌ Tables
+- 🟡 Tables
 - 🟡 Section properties
 - 🟡 Default section properties / document-level formatting and settings
 - 🟡 Fields (supported but additional work is needed for some complex fields and form fields)

src/DocSharp.Docx/RtfToDocx/RtfToDocxConverter.Table.cs

@@ -90,6 +90,20 @@ private void EndTableRow()
             return;
         }
         var table = container.LastChild as Table ?? container.AppendChild(new Table());
+        
+        // We have to add GridSpan to cells based on their width and the number of cells, 
+        // otherwise horizontally merged cells do not behave as expected, even if they width is increased.
+        // The following approach is not ideal, but I can't think of a better way at this time.
+        int minWidth = Math.Max(pendingTableRow.Elements<TableCell>().Min(c => (c.TableCellProperties?.TableCellWidth?.Width?.Value).ToIntInvariant(0)), 1);
+        foreach (var cell in pendingTableRow.Elements<TableCell>())
+        {
+            int cellWidth = Math.Max((cell.TableCellProperties?.TableCellWidth?.Width?.Value).ToIntInvariant(0), 1);
+            int gridSpan = (int)Math.Round((double)cellWidth / minWidth, MidpointRounding.AwayFromZero);
+            // Grid span is always at least 1. 
+            if (gridSpan > 1)
+                cell.TableCellProperties!.GridSpan = new GridSpan() { Val = gridSpan };
+        }
+
         table.AppendChild(pendingTableRow);
         pendingTableRow = null;
         // Do not reset row properties and exceptions here, as they can be inherited by subsequent rows until a new trowd control word is encountered
@@ -139,6 +153,7 @@ private bool ProcessTableControlWord(RtfControlWord cw)
                 currentTableRowExceptions = new();
                 inTableRowDefinition = true;
                 cellIndex = 0;
+                cellx = 0;
 				return true;
             case "row":
             case "nestrow":