give the "Set as Player Ship" button exclusive semantics in both editors

code/missioneditor/common.cpp

@@ -153,16 +153,64 @@ void ensure_valid_player_start_shipnum()
 	// nothing to do if the current player start is still valid
 	if (Player_start_shipnum >= 0 && Player_start_shipnum < MAX_SHIPS
 		&& Ships[Player_start_shipnum].objnum >= 0
-		&& Objects[Ships[Player_start_shipnum].objnum].type == OBJ_START) {
+		&& Objects[Ships[Player_start_shipnum].objnum].type == OBJ_START)
+	{
 		return;
 	}
 
 	// otherwise repoint to the first remaining player start, or -1 if there are none
 	Player_start_shipnum = -1;
-	for (auto *objp : list_range(&obj_used_list)) {
-		if (objp->type == OBJ_START) {
+	for (auto *objp : list_range(&obj_used_list))
+	{
+		if (objp->type == OBJ_START)
+		{
 			Player_start_shipnum = objp->instance;
 			break;
 		}
 	}
 }
+
+bool set_single_player_start(int objnum)
+{
+	if (objnum < 0 || objnum >= MAX_OBJECTS
+		|| (Objects[objnum].type != OBJ_SHIP && Objects[objnum].type != OBJ_START))
+	{
+		Assertion(false, "set_single_player_start() called for object %d, which is not a ship", objnum);
+		return false;
+	}
+
+	bool changed = false;
+
+	for (auto *objp : list_range(&obj_used_list))
+	{
+		if (objp->type != OBJ_SHIP && objp->type != OBJ_START)
+			continue;
+
+		if (OBJ_INDEX(objp) == objnum)
+		{
+			// set as player ship
+			if (objp->type != OBJ_START)
+			{
+				objp->type = OBJ_START;
+				Player_starts++;
+				changed = true;
+			}
+			objp->flags.set(Object::Object_Flags::Player_ship);
+		}
+		else
+		{
+			// set as regular ship
+			if (objp->type == OBJ_START)
+			{
+				objp->type = OBJ_SHIP;
+				Player_starts--;
+				changed = true;
+			}
+			objp->flags.remove(Object::Object_Flags::Player_ship);
+		}
+	}
+
+	ensure_valid_player_start_shipnum();
+
+	return changed;
+}

code/missioneditor/common.h

@@ -53,3 +53,8 @@ void generate_weaponry_usage_list_wing(int wing_num, int* arr);
 // first remaining player start in the mission (or -1 if there are none).  Call this after
 // changing a ship to or from an OBJ_START via demotion, deletion, etc.
 void ensure_valid_player_start_shipnum();
+
+// Make the given object the sole player start, per single-player semantics: promote it to
+// OBJ_START, demote every other player start to OBJ_SHIP, adjust Player_starts to match,
+// and repoint Player_start_shipnum.  Returns true if anything changed.
+bool set_single_player_start(int objnum);

fred2/shipeditordlg.cpp

@@ -2431,40 +2431,22 @@ void CShipEditorDlg::OnSetAsPlayerShip()
 		return;
 	}
 
-	// since this is single player, clear all player ships and set only this one
-	object *objp = GET_FIRST(&obj_used_list);
-	while (objp != END_OF_LIST(&obj_used_list))
+	// find the selected ship; there should only be one
+	object *marked_objp = nullptr;
+	for (auto objp: list_range(&obj_used_list))
 	{
-		if ((objp->type == OBJ_SHIP) || (objp->type == OBJ_START))
+		if (((objp->type == OBJ_SHIP) || (objp->type == OBJ_START)) && objp->flags[Object::Object_Flags::Marked])
 		{
-			if (objp->flags[Object::Object_Flags::Marked])	// there should only be one selected ship
-			{
-				// set as player ship
-				if (objp->type != OBJ_START)
-				{
-					Player_starts++;
-					set_modified();
-				}
-				objp->type = OBJ_START;
-				objp->flags.set(Object::Object_Flags::Player_ship);
-			}
-			else
-			{
-				// set as regular ship
-				if (objp->type == OBJ_START)
-				{
-					Player_starts--;
-					set_modified();
-				}
-				objp->type = OBJ_SHIP;
-				objp->flags.remove(Object::Object_Flags::Player_ship);
-			}
+			marked_objp = objp;
+			break;
 		}
-		objp = GET_NEXT(objp);
 	}
+	if (marked_objp == nullptr)
+		return;
 
-	// fix up Player_start_shipnum if it became invalid
-	ensure_valid_player_start_shipnum();
+	// since this is single player, clear all player ships and set only this one
+	if (set_single_player_start(OBJ_INDEX(marked_objp)))
+		set_modified();
 
 	// finally set editor dialog
 	m_player_ship.SetCheck(1);

qtfred/src/mission/dialogs/ShipEditor/ShipEditorDialogModel.cpp

@@ -1162,6 +1162,24 @@ bool ShipEditorDialogModel::getPlayer() const
 	return _isPlayerShip;
 }
 
+void ShipEditorDialogModel::makeSolePlayerStart()
+{
+	// the ship being edited, which is either a player start or a regular ship
+	int shipnum = (_playerShipIndex >= 0) ? _playerShipIndex : _singleShip;
+	if (shipnum < 0)
+		return;
+
+	// since this is single player, clear all player ships and set only this one
+	if (set_single_player_start(Ships[shipnum].objnum))
+	{
+		setModified();
+		_editor->missionChanged();
+	}
+
+	_isPlayerShip = true;
+	modelChanged();
+}
+
 void ShipEditorDialogModel::setRespawn(const int value)
 {
 	if (_respawnPriority == value)

qtfred/src/mission/dialogs/ShipEditor/ShipEditorDialogModel.h

@@ -59,6 +59,7 @@ class ShipEditorDialogModel : public AbstractDialogModel {
 
 	void setPlayer(bool isPlayer);
 	bool getPlayer() const;
+	void makeSolePlayerStart();
 
 	void setRespawn(int respawn);
 	int getRespawn() const;

qtfred/src/ui/dialogs/ShipEditor/ShipEditorDialog.cpp

@@ -641,7 +641,7 @@ void ShipEditorDialog::on_textureReplacementButton_clicked()
 
 void ShipEditorDialog::on_playerShipButton_clicked()
 {
-	_model->setPlayer(true);
+	_model->makeSolePlayerStart();
 }
 void ShipEditorDialog::on_altShipClassButton_clicked()
 {