Fix slots window not being horizontally resizable

See: #515

src/tracerestrict_gui.cpp

@@ -3587,15 +3587,14 @@ static const NWidgetPart _nested_slot_widgets[] = {
 				NWidget(NWID_SELECTION, INVALID_COLOUR, WID_TRSL_FILTER_BY_CARGO_SEL),
 					NWidget(WWT_DROPDOWN, COLOUR_GREY, WID_TRSL_FILTER_BY_CARGO), SetMinimalSize(167, 12), SetDataTip(STR_JUST_STRING, STR_TOOLTIP_FILTER_CRITERIA),
 				EndContainer(),
-				NWidget(WWT_PANEL, COLOUR_GREY), SetMinimalSize(12, 12), SetResize(1, 0), EndContainer(),
+				NWidget(WWT_PANEL, COLOUR_GREY), SetMinimalSize(0, 12), SetResize(1, 0), EndContainer(),
 			EndContainer(),
 			NWidget(NWID_HORIZONTAL),
 				NWidget(WWT_MATRIX, COLOUR_GREY, WID_TRSL_LIST_VEHICLE), SetMinimalSize(248, 0), SetMatrixDataTip(1, 0, STR_VEHICLE_LIST_TRAIN_LIST_TOOLTIP), SetResize(1, 1), SetFill(1, 0), SetScrollbar(WID_TRSL_LIST_VEHICLE_SCROLLBAR),
 				NWidget(NWID_VSCROLLBAR, COLOUR_GREY, WID_TRSL_LIST_VEHICLE_SCROLLBAR),
 			EndContainer(),
-			NWidget(WWT_PANEL, COLOUR_GREY), SetMinimalSize(1, 0), SetFill(1, 1), SetResize(1, 0), EndContainer(),
 			NWidget(NWID_HORIZONTAL),
-				NWidget(WWT_PANEL, COLOUR_GREY), SetFill(1, 1), EndContainer(),
+				NWidget(WWT_PANEL, COLOUR_GREY), SetFill(1, 1), SetResize(1, 0), EndContainer(),
 				NWidget(WWT_RESIZEBOX, COLOUR_GREY),
 			EndContainer(),
 		EndContainer(),