Fix library tree column widths when tree is refreshed

GTK only calculates new widths once the tree is displayed,
so calling Refresh after updating the tree but before display
will return a column width of 0. Instead, use saved column
widths if an invalid width was returned.

Fixes https://gitlab.com/kicad/code/kicad/issues/4837

common/lib_tree_model_adapter.cpp

@@ -403,8 +403,22 @@ void LIB_TREE_MODEL_ADAPTER::RefreshTree()
     // user's scroll position (which re-attaching or deleting/re-inserting columns does).
     static int walk = 1;
 
-    m_col_part->SetWidth( m_col_part->GetWidth() + walk );
-    m_col_desc->SetWidth( m_col_desc->GetWidth() - walk );
+    int partWidth = m_col_part->GetWidth();
+    int descWidth = m_col_desc->GetWidth();
+
+    // Only use the widths read back if they are non-zero.
+    // GTK returns the displayed width of the column, which is not calculated immediately
+    if( descWidth > 0 )
+    {
+        m_colWidths[PART_COL] = partWidth;
+        m_colWidths[DESC_COL] = descWidth;
+    }
+
+    m_colWidths[PART_COL] += walk;
+    m_colWidths[DESC_COL] -= walk;
+
+    m_col_part->SetWidth( m_colWidths[PART_COL] );
+    m_col_desc->SetWidth( m_colWidths[DESC_COL] );
     walk = -walk;
 }