Let listboxs update its contents when adding rows.

Before only remove reduced the size, now the adding also updates contents.
This should reduce the number of layout updates.

changelog

@@ -55,6 +55,7 @@ Version 1.13.0-dev:
    * Made orb and minmap colors configurable by the game preferences.
    * Remove 'allow_new_game=no' entries from random map new game list
    * Fixed bug #22095: An assertion failure in the gamestate inspector.
+   * Changed: A listbox can now update its size when rows are added.
  * WML engine:
    * Added customizable recall costs for unit types and individual units,
      using the new recall_cost attribute in [unit_type] and [unit].

src/gui/widgets/listbox.cpp

@@ -66,8 +66,10 @@ tlistbox::tlistbox(const bool has_minimum,
 void tlistbox::add_row(const string_map& item, const int index)
 {
 	assert(generator_);
-	generator_->create_item(
+	const tgrid& row = generator_->create_item(
 			index, list_builder_, item, callback_list_item_clicked);
+
+	resize_content(row);
 }
 
 void
@@ -75,8 +77,10 @@ tlistbox::add_row(const std::map<std::string /* widget id */, string_map>& data,
 				  const int index)
 {
 	assert(generator_);
-	generator_->create_item(
+	const tgrid& row = generator_->create_item(
 			index, list_builder_, data, callback_list_item_clicked);
+
+	resize_content(row);
 }
 
 void tlistbox::remove_row(const unsigned row, unsigned count)
@@ -315,6 +319,26 @@ void tlistbox::resize_content(const int width_modification,
 	}
 }
 
+void tlistbox::resize_content(const twidget& row)
+{
+	if(row.get_visible() == tvisible::invisible) {
+		return;
+	}
+
+	DBG_GUI_L << LOG_HEADER << " current size " << content_grid()->get_size()
+			  << " row size " << row.get_best_size() << ".\n";
+
+	const tpoint content = content_grid()->get_size();
+	tpoint size = row.get_best_size();
+	if(size.x < content.x) {
+		size.x = 0;
+	} else {
+		size.x -= content.x;
+	}
+
+	resize_content(size.x, size.y);
+}
+
 void tlistbox::layout_children()
 {
 	layout_children(false);

src/gui/widgets/listbox.hpp

@@ -303,6 +303,15 @@ private:
 	void resize_content(const int width_modification,
 						const int height_modification);
 
+	/**
+	 * Resizes the content.
+	 *
+	 * The resize happens when a new row is added to the contents.
+	 *
+	 * @param row                 The new row added to the listbox.
+	 */
+	void resize_content(const twidget& row);
+
 	/** Layouts the children if needed. */
 	void layout_children(const bool force);