diff -r 485ef234c1e2 src/group_gui.cpp
--- a/src/group_gui.cpp	Sat May 22 15:37:53 2010 +0200
+++ b/src/group_gui.cpp	Sat May 22 17:17:26 2010 +0200
@@ -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
+	GroupID group_over;    ///< Group over which a vehicle is dragged, INVALID_GROUP if none
 
 	/**
 	 * (Re)Build the group list.
@@ -194,6 +195,22 @@
 		return r;
 	}
 
+	/**
+	 * Marks dirty the widget containing the currently highlighted group (#this->group_over).
+	 */
+	void SetHighlightedGroupWidgetDirty()
+	{
+		if (this->group_over == INVALID_GROUP) return;
+
+		if (IsAllGroupID(this->group_over)) {
+			this->SetWidgetDirty(GRP_WIDGET_ALL_VEHICLES);
+		} else if (IsDefaultGroupID(this->group_over)) {
+			this->SetWidgetDirty(GRP_WIDGET_DEFAULT_VEHICLES);
+		} else {
+			this->SetWidgetDirty(GRP_WIDGET_LIST_GROUP);
+		}
+	}
+
 public:
 	VehicleGroupWindow(const WindowDesc *desc, WindowNumber window_number) : BaseVehicleListWindow()
 	{
@@ -211,6 +228,7 @@
 		this->group_sel = ALL_GROUP;
 		this->vehicle_sel = INVALID_VEHICLE;
 		this->group_rename = INVALID_GROUP;
+		this->group_over = INVALID_GROUP;
 
 		const Owner owner = (Owner)GB(window_number, 0, 8);
 		this->vehicles.SetListing(*this->sorting);
@@ -381,11 +399,21 @@
 	{
 		switch (widget) {
 			case GRP_WIDGET_ALL_VEHICLES:
+				if (!IsAllGroupID(this->group_sel) && IsAllGroupID(this->group_over)) {
+					/* This group isn't selected and a vehicle is dragged over it, highlight it. */
+					GfxFillRect(r.left + WD_FRAMERECT_LEFT, r.top + WD_FRAMERECT_TOP + 1, r.right - WD_FRAMERECT_RIGHT,
+								r.bottom - WD_FRAMERECT_BOTTOM, _colour_gradient[COLOUR_GREY][7]);
+				}
 				DrawString(r.left + WD_FRAMERECT_LEFT + 8, r.right - WD_FRAMERECT_RIGHT - 8, r.top + WD_FRAMERECT_TOP + 1,
 						STR_GROUP_ALL_TRAINS + this->vehicle_type, IsAllGroupID(this->group_sel) ? TC_WHITE : TC_BLACK);
 				break;
 
 			case GRP_WIDGET_DEFAULT_VEHICLES:
+				if (!IsDefaultGroupID(this->group_sel) && IsDefaultGroupID(this->group_over)) {
+					/* This group isn't selected and a vehicle is dragged over it, highlight it. */
+					GfxFillRect(r.left + WD_FRAMERECT_LEFT, r.top + WD_FRAMERECT_TOP + 1, r.right - WD_FRAMERECT_RIGHT,
+								r.bottom - WD_FRAMERECT_BOTTOM, _colour_gradient[COLOUR_GREY][7]);
+				}
 				DrawString(r.left + WD_FRAMERECT_LEFT + 8, r.right - WD_FRAMERECT_RIGHT - 8, r.top + WD_FRAMERECT_TOP + 1,
 						STR_GROUP_DEFAULT_TRAINS + this->vehicle_type, IsDefaultGroupID(this->group_sel) ? TC_WHITE : TC_BLACK);
 				break;
@@ -398,6 +426,12 @@
 
 					assert(g->owner == this->owner);
 
+					if (this->group_sel != g->index && this->group_over == g->index) {
+						/* This group isn't selected and a vehicle is dragged over it, highlight it. */
+						GfxFillRect(r.left + WD_FRAMERECT_LEFT, y1, r.right - WD_FRAMERECT_RIGHT,
+									y1 + FONT_HEIGHT_NORMAL - 1, _colour_gradient[COLOUR_GREY][7]);
+					}
+
 					/* draw the selected group in white, else we draw it in black */
 					SetDParam(0, g->index);
 					DrawString(r.left + WD_FRAMERECT_LEFT + 8, r.right - WD_FRAMERECT_RIGHT - 8, y1, STR_GROUP_NAME, (this->group_sel == g->index) ? TC_WHITE : TC_BLACK);
@@ -537,6 +571,7 @@
 				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 +581,7 @@
 				const VehicleID vindex = this->vehicle_sel;
 
 				this->vehicle_sel = INVALID_VEHICLE;
+				this->group_over = INVALID_GROUP;
 
 				this->SetDirty();
 
@@ -564,6 +600,7 @@
 				const VehicleID vindex = this->vehicle_sel;
 
 				this->vehicle_sel = INVALID_VEHICLE;
+				this->group_over = INVALID_GROUP;
 
 				this->SetDirty();
 
@@ -655,9 +692,47 @@
 	{
 		/* abort drag & drop */
 		this->vehicle_sel = INVALID_VEHICLE;
+		this->SetHighlightedGroupWidgetDirty();
+		this->group_over = INVALID_GROUP;
 		this->SetWidgetDirty(GRP_WIDGET_LIST_VEHICLE);
 	}
 
+	virtual void OnMouseDrag(Point pt, int widget)
+	{
+		if (this->vehicle_sel != INVALID_VEHICLE) {
+			/* A vehicle is dragged over… */
+			GroupID new_group_over = INVALID_GROUP;
+			switch(widget) {
+				case GRP_WIDGET_ALL_VEHICLES:
+					new_group_over = ALL_GROUP; // …the 'All' group.
+					break;
+
+				case GRP_WIDGET_DEFAULT_VEHICLES: // …the 'Default' group.
+					new_group_over = DEFAULT_GROUP;
+					break;
+
+				case GRP_WIDGET_LIST_GROUP: { // …the list of custom groups.
+					uint group = (pt.y - this->GetWidget<NWidgetBase>(widget)->pos_y) / ((int) this->tiny_step_height);
+
+					if (group < this->vscroll2.GetCapacity()) {
+						group += this->vscroll2.GetPosition();
+
+						if (group < this->groups.Length()) { // ignore empty cells.
+							new_group_over = this->groups[group]->index;
+						}
+					}
+					break;
+				}
+			}
+
+			if (new_group_over == this->group_over) return;
+
+			this->SetHighlightedGroupWidgetDirty(); // Marks dirty the widget of the old highlighted group.
+			this->group_over = new_group_over;
+			this->SetHighlightedGroupWidgetDirty(); // Marks dirty the widget of the new highlighted group.
+		}
+	}
+
 	void ShowRenameGroupWindow(GroupID group, bool empty)
 	{
 		assert(Group::IsValidID(group));