diff -r f6e30cce6f5f src/newgrf_gui.cpp
--- a/src/newgrf_gui.cpp	Mon May 24 09:23:03 2010 +0200
+++ b/src/newgrf_gui.cpp	Mon May 24 09:38:43 2010 +0200
@@ -488,6 +488,8 @@
 				NWidgetBase *nw = this->GetWidget<NWidgetBase>(SNGRFS_FILE_LIST);
 				uint i = (pt.y - nw->pos_y) / nw->resize_y + this->vscroll.GetPosition();
 
+				if (i + 1 == this->vscroll.GetCount()) break; // Do nothing if the click occurs on the empty line at the end of the list
+
 				GRFConfig *c;
 				for (c = this->actives; c != NULL && i > 0; c = c->next, i--) {}
 
@@ -544,7 +546,13 @@
 				}
 				this->InvalidateData();
 				this->DeleteChildWindows(WC_QUERY_STRING); // Remove the parameter query window
-				if (click_count == 1) break;
+				if (click_count == 1) {
+					if (this->avail_sel != NULL) {
+						/* Activate drag and drop */
+						SetObjectToPlaceWnd(SPR_CURSOR_MOUSE, PAL_NONE, HT_DRAG, this);
+					}
+					break;
+				}
 			}
 			/* Fall through with double click. */
 			case SNGRFS_ADD: {
@@ -647,50 +655,88 @@
 
 	virtual void OnDragDrop(Point pt, int widget)
 	{
-		if (widget == SNGRFS_FILE_LIST && this->active_sel != NULL) {
-			GRFConfig *from = this->active_sel;
-			/* Gets the selected file offset in the active list and its predecessor. */
-			GRFConfig *from_prev = NULL;
-			uint from_pos = 0;
-			for (GRFConfig *cursor = this->actives; cursor != NULL; cursor = cursor->next) {
-				if (cursor == from) break;
+		if (widget == SNGRFS_FILE_LIST) {
+			if (this->active_sel != NULL) { // Move NewGRF file inside active list
+				GRFConfig *from = this->active_sel;
+				/* Gets the selected file offset in the active list and its predecessor. */
+				GRFConfig *from_prev = NULL;
+				uint from_pos = 0;
+				for (GRFConfig *cursor = this->actives; cursor != NULL; cursor = cursor->next) {
+					if (cursor == from) break;
 
-				from_prev = cursor;
-				from_pos++;
-			}
-
-			/* Gets the drag-and-drop destination offset. */
-			NWidgetBase *nw = this->GetWidget<NWidgetBase>(SNGRFS_FILE_LIST);
-			uint to_pos = (pt.y - nw->pos_y) / nw->resize_y + this->vscroll.GetPosition();
-
-			if (from_pos != to_pos) { // Don't move NewGRF file over itself
-				/* Gets destination file and its predecessor. */
-				GRFConfig *to = this->actives;
-				GRFConfig *to_prev = NULL;
-				for (uint i = 0; to != NULL && i < to_pos; to_prev = to, to = to->next, i++) {}
-
-				/* Detach NewGRF file of its original position. */
-				if (from_prev == NULL) { // Detach from start of list
-					this->actives = this->actives->next;
-				} else {
-					from_prev->next = from->next;
+					from_prev = cursor;
+					from_pos++;
 				}
 
+				/* Gets the drag-and-drop destination offset. */
+				NWidgetBase *nw = this->GetWidget<NWidgetBase>(SNGRFS_FILE_LIST);
+				uint to_pos = (pt.y - nw->pos_y) / nw->resize_y + this->vscroll.GetPosition();
+				if (to_pos + 1 == this->vscroll.GetCount()) to_pos--; // Active list contains an empty line at end, skip it
+
+				if (from_pos != to_pos) { // Don't move NewGRF file over itself
+					/* Gets destination file and its predecessor. */
+					GRFConfig *to = this->actives;
+					GRFConfig *to_prev = NULL;
+					for (uint i = 0; to != NULL && i < to_pos; to_prev = to, to = to->next, i++) {}
+
+					/* Detach NewGRF file of its original position. */
+					if (from_prev == NULL) { // Detach from start of list
+						this->actives = this->actives->next;
+					} else {
+						from_prev->next = from->next;
+					}
+
+					/* Attach NewGRF file to its new position. */
+					if (from_pos < to_pos) { // Move down NewGRF file
+						from->next = to->next;
+						to->next = from;
+					} else if (to_prev == NULL) { // Move up at start of the list
+						this->actives = from;
+						from->next = to;
+					} else { // Move up inside list
+						from->next = to;
+						to_prev->next = from;
+					}
+
+					this->vscroll.ScrollTowards(to_pos);
+					this->preset = -1;
+					this->InvalidateData();
+				}
+			} else if (this->avail_sel != NULL) { // Insert available NewGRF file into active list
+				/* Check for duplicate grfid. */
+				for (GRFConfig *cursor = this->actives; cursor != NULL; cursor = cursor->next) {
+					if (cursor->ident.grfid == this->avail_sel->ident.grfid) {
+						ShowErrorMessage(STR_NEWGRF_DUPLICATE_GRFID, INVALID_STRING_ID, WL_INFO);
+						ResetObjectToPlace();
+						return;
+					}
+				}
+
+				GRFConfig *from = DuplicateGRFConfig(this->avail_sel);
+
+				/* Gets the drag-and-drop destination offset. */
+				NWidgetBase *nw = this->GetWidget<NWidgetBase>(SNGRFS_FILE_LIST);
+				uint to_pos = (pt.y - nw->pos_y) / nw->resize_y + this->vscroll.GetPosition();
+
 				/* Attach NewGRF file to its new position. */
-				if (from_pos < to_pos) { // Move down NewGRF file
-					from->next = to->next;
-					to->next = from;
-				} else if (to_prev == NULL) { // Move up at start of the list
+				if (to_pos == 0 || this->actives == NULL) { // Insert at start of list
+					from->next = this->actives;
 					this->actives = from;
-					from->next = to;
-				} else { // Move up inside list
-					from->next = to;
+				} else {
+					/* Gets file before the destination position. */
+					GRFConfig *to_prev = this->actives;
+					for (uint i = 1; i < to_pos && to_prev->next != NULL; i++, to_prev = to_prev->next) {}
+
+					/* Insert NewGRF file inside list. */
+					from->next = to_prev->next;
 					to_prev->next = from;
 				}
 
-				this->vscroll.ScrollTowards(to_pos);
-				this->preset = -1;
-				this->InvalidateData();
+				this->active_sel = from;
+				this->avail_sel = NULL;
+				this->avail_pos = -1;
+				this->avails.ForceRebuild();
+				this->InvalidateData(2);
 			}
 		}
 
@@ -800,7 +846,7 @@
 				for (const GRFConfig *c = this->actives; c != NULL; c = c->next, i++) {}
 
 				this->vscroll.SetCapacityFromWidget(this, SNGRFS_FILE_LIST);
-				this->vscroll.SetCount(i);
+				this->vscroll.SetCount(i + 1); // Reserve an empty space at end of list: used to drag an available file at end of active list
 
 				this->vscroll2.SetCapacityFromWidget(this, SNGRFS_AVAIL_LIST);
 				if (this->avail_pos >= 0) this->vscroll2.ScrollTowards(this->avail_pos);