Fix an assertion failure in the GUI2 placement.

If a vertical scrollbar is added to a widget its size is recalculated.
Unfortunalty the determination of the minimum width didn't take the
horizontal scrollbar in account. This lead to the container to be too
small for a widget with horizontal and vertical scrollbar causing the
placement to fail.

Fixes bug #22095, possibly #15615 and #21785.

There are still a few issues remaining, but it seems they only cause the
code to behave less efficient. The fix for this issue seems to be more
intrusive and will only be committed to master.

changelog

@@ -17,6 +17,8 @@ Version 1.11.15+dev:
    * Fixed error messages about missing UI elements.
  * Language and i18n:
    * Updated translations:
+ * User interface:
+   * Fixed bug #22095: An assertion failure in the gamestate inspector.
  * WML engine:
    * Fixed a regression in 1.11.14 causing WML parser/preprocessor errors to
      not interrupt the game load sequence or display an error message in-game,

src/gui/widgets/scrollbar_container.cpp

@@ -263,7 +263,17 @@ void tscrollbar_container::request_reduce_width(const unsigned maximum_width)
 	horizontal_scrollbar_grid_->set_visible(twidget::tvisible::visible);
 	size = get_best_size();
 
-	const tpoint scrollbar_size = horizontal_scrollbar_grid_->get_best_size();
+	tpoint scrollbar_size = horizontal_scrollbar_grid_->get_best_size();
+
+	/*
+	 * If the vertical bar is not invisible it's size needs to be added to the
+	 * minimum size.
+	 */
+	if(vertical_scrollbar_grid_->get_visible()
+	   != twidget::tvisible::invisible) {
+
+		scrollbar_size.x += vertical_scrollbar_grid_->get_best_size().x;
+	}
 
 	// If showing the scrollbar increased the width, hide and abort.
 	if(horizontal_scrollbar_mode_ == auto_visible_first_run && scrollbar_size.x