diff -r 63bc2c41dce2 src/group_gui.cpp
--- a/src/group_gui.cpp	Fri Mar 19 13:54:47 2010 +0100
+++ b/src/group_gui.cpp	Fri Mar 19 13:54:57 2010 +0100
@@ -148,6 +148,7 @@
 	GroupID group_rename;  ///< Group being renamed, INVALID_GROUP if none
 	GUIGroupList groups;   ///< List of groups
 	uint tiny_step_height; ///< Step height for the group list
+	uint16 group_over;     ///< A vehicle is currently dragged over this group
 
 	/**
 	 * (Re)Build the group list.
@@ -194,9 +195,35 @@
 		return r;
 	}
 
+	/** Highlight the given group cell: draw an horizontal arrow at the left (or right
+	    for rtl languages) and at the 'cell_offset'th position. */
+	void HighlightGroupCell(const Rect &r, uint cell_offset = 0) const
+	{
+		bool rtl = _dynlang.text_dir == TD_RTL;
+		SpriteID sprite = rtl ? SPR_ARROW_LEFT : SPR_ARROW_RIGHT;
+		Dimension sprite_size = GetSpriteSize(sprite);
+		uint x = rtl ? r.right - WD_FRAMERECT_RIGHT - sprite_size.width : r.left + WD_FRAMERECT_LEFT;
+		uint y = r.top + cell_offset * this->tiny_step_height + (this->tiny_step_height / 2) - (sprite_size.height / 2);
+
+		DrawSprite(sprite, PAL_NONE, x, y);
+	}
+
+	/** Sets dirty the group widget currently under a dragged vehicle. */
+	void SetHighlightedWidgetDirty()
+	{
+		switch (this->group_over) {
+			case INVALID_GROUP: break;
+			case ALL_GROUP: this->SetWidgetDirty(GRP_WIDGET_ALL_VEHICLES); break;
+			case DEFAULT_GROUP: this->SetWidgetDirty(GRP_WIDGET_DEFAULT_VEHICLES); break;
+			default: this->SetWidgetDirty(GRP_WIDGET_LIST_GROUP); break;
+		}
+	}
+
 public:
 	VehicleGroupWindow(const WindowDesc *desc, WindowNumber window_number) : BaseVehicleListWindow()
 	{
+		this->group_over = INVALID_GROUP;
+
 		this->CreateNestedTree(desc);
 
 		this->vehicle_type = (VehicleType)GB(window_number, 11, 5);
@@ -383,11 +410,13 @@
 			case GRP_WIDGET_ALL_VEHICLES:
 				DrawString(r.left + WD_FRAMERECT_LEFT + 8, r.right - WD_FRAMERECT_RIGHT - 8, r.top + WD_MATRIX_TOP,
 						STR_GROUP_ALL_TRAINS + this->vehicle_type, IsAllGroupID(this->group_sel) ? TC_WHITE : TC_BLACK);
+				if (IsAllGroupID(this->group_over)) this->HighlightGroupCell(r);
 				break;
 
 			case GRP_WIDGET_DEFAULT_VEHICLES:
 				DrawString(r.left + WD_FRAMERECT_LEFT + 8, r.right - WD_FRAMERECT_RIGHT - 8, r.top + WD_MATRIX_TOP,
 						STR_GROUP_DEFAULT_TRAINS + this->vehicle_type, IsDefaultGroupID(this->group_sel) ? TC_WHITE : TC_BLACK);
+				if (IsDefaultGroupID(this->group_over)) this->HighlightGroupCell(r);
 				break;
 
 			case GRP_WIDGET_LIST_GROUP: {
@@ -406,6 +435,8 @@
 					SetDParam(0, g->num_vehicle);
 					DrawString(r.left + WD_FRAMERECT_LEFT, r.right - WD_FRAMERECT_RIGHT, y1 + 1, STR_TINY_COMMA, (this->group_sel == g->index) ? TC_WHITE : TC_BLACK, SA_RIGHT);
 
+					if (this->group_over == g->index) this->HighlightGroupCell(r, i - this->vscroll2.GetPosition());
+
 					y1 += this->tiny_step_height;
 				}
 				break;
@@ -537,6 +568,7 @@
 				DoCommandP(0, DEFAULT_GROUP, this->vehicle_sel, CMD_ADD_VEHICLE_GROUP | CMD_MSG(STR_ERROR_GROUP_CAN_T_ADD_VEHICLE));
 
 				this->vehicle_sel = INVALID_VEHICLE;
+				this->group_over = INVALID_GROUP;
 
 				this->SetDirty();
 				break;
@@ -546,6 +578,7 @@
 				const VehicleID vindex = this->vehicle_sel;
 
 				this->vehicle_sel = INVALID_VEHICLE;
+				this->group_over = INVALID_GROUP;
 
 				this->SetDirty();
 
@@ -564,6 +597,7 @@
 				const VehicleID vindex = this->vehicle_sel;
 
 				this->vehicle_sel = INVALID_VEHICLE;
+				this->group_over = INVALID_GROUP;
 
 				this->SetDirty();
 
@@ -655,9 +689,42 @@
 	{
 		/* abort drag & drop */
 		this->vehicle_sel = INVALID_VEHICLE;
+		this->SetHighlightedWidgetDirty();
+		this->group_over = INVALID_GROUP;
 		this->SetWidgetDirty(GRP_WIDGET_LIST_VEHICLE);
 	}
 
+	virtual void OnMouseDrag(Point pt, int widget)
+	{
+		if (this->vehicle_sel != INVALID_VEHICLE) {
+			/* We are dragging a vehicle */
+
+			uint16 new_group_over = INVALID_GROUP;
+			switch (widget) {
+				default: new_group_over = INVALID_GROUP; break;
+				case GRP_WIDGET_DEFAULT_VEHICLES: new_group_over = DEFAULT_GROUP; break;
+				case GRP_WIDGET_ALL_VEHICLES: new_group_over = ALL_GROUP; break;
+				case GRP_WIDGET_LIST_GROUP: {
+					uint16 id_g = (pt.y - this->GetWidget<NWidgetBase>(widget)->pos_y) / (int)this->tiny_step_height;
+
+					if (id_g < this->vscroll2.GetCapacity()) {
+						id_g += this->vscroll2.GetPosition();
+
+						if (id_g < this->groups.Length()) { // over a valid user group
+							new_group_over = this->groups[id_g]->index;
+						}
+					}
+				} break;
+			}
+
+			if (new_group_over == this->group_over) return;
+
+			this->SetHighlightedWidgetDirty();
+			this->group_over = new_group_over;
+			this->SetHighlightedWidgetDirty();
+		}
+	}
+
 	void ShowRenameGroupWindow(GroupID group, bool empty)
 	{
 		assert(Group::IsValidID(group));