Click again on a member of a selected group to pick it on its own

Clicking an item of a group selects the whole group (#1070). Clicking
again on one of its items is meant to select just that item, to edit it
on its own -- what discussion #1070 proposed -- but the second click
selected the whole group again: Qt left only the clicked item selected on
release, and the group completion pulled the others back in.

A press on a member of a group that is selected whole now notes that
member (ItemGroups::memberToPick(), which also finds the member when the
click lands on a symbol's own text). If the click ends without a drag and
Qt has left only that member selected, the selection stays so. A drag
still moves the whole group; Ctrl+click keeps its meaning; a group of one
is not picked from.

In the GUI, on two grouped texts: one click then Delete removes both
(master and this); click, click again, Delete removes only the clicked
text here, both on master; dragging after one click moves both texts by
the same amount on both. tst_itemgroups: 5 new checks; removing the
whole-group or the group-of-one condition fails one each. ctest 24/24.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/qttest/tst_itemgroups.cpp

@@ -121,6 +121,48 @@ private slots:
 		b = nullptr;
 	}
 
+	// A second click on a member of a group selected whole picks that member
+	// out; a click on a member of a group not selected whole does not.
+	void aMemberOfAWholeGroupCanBePicked()
+	{
+		select({a, b});
+		QCOMPARE(ItemGroups::memberToPick(a), a);
+		QCOMPARE(ItemGroups::memberToPick(b), b);
+	}
+
+	void aMemberOfAPartlySelectedGroupIsNotPicked()
+	{
+		select({a});       // after one member was picked
+		QCOMPARE(ItemGroups::memberToPick(a), nullptr);
+		select({});
+		QCOMPARE(ItemGroups::memberToPick(a), nullptr);
+	}
+
+	void anUngroupedItemIsNotPicked()
+	{
+		select({c});
+		QCOMPARE(ItemGroups::memberToPick(c), nullptr);
+		QCOMPARE(ItemGroups::memberToPick(nullptr), nullptr);
+	}
+
+	void aGroupOfOneIsNotPicked()
+	{
+		ItemGroups::setGroup(e, QUuid());   // g2 is now d alone
+		select({d});
+		QCOMPARE(ItemGroups::memberToPick(d), nullptr);
+	}
+
+	// A click lands on a symbol's own text, not on the symbol: the member is
+	// the nearest grouped ancestor.
+	void aClickOnAMembersChildPicksTheMember()
+	{
+		auto child = new QGraphicsRectItem(0, 0, 2, 2, a);
+		QCOMPARE(ItemGroups::groupedItem(child), a);
+		select({a, b});
+		QCOMPARE(ItemGroups::memberToPick(child), a);
+		QCOMPARE(ItemGroups::groupedItem(c), nullptr);
+	}
+
 	void xmlRoundTrip()
 	{
 		QDomDocument doc;