SCI32: Change plane and screen item sorting algorithm

SSCI's last resort comparison here was to compare the object IDs
of planes & screen items, but this randomly breaks (at least) the
"you have died" dialog at the end of Phant1, and text & buttons
in Hoyle5, even in SSCI itself.

This commit changes last resort comparison to use a monotonically
increasing ID instead, which keeps objects with identical priority
& z-index in creation order when compared.

Fixes Trac#9585.

1 files changed, 37 insertions, 2 deletions

diff --git a/engines/sci/graphics/screen_item32.h b/engines/sci/graphics/screen_item32.h
index 009b608f18..c2c4e43358 100644
--- a/engines/sci/graphics/screen_item32.h
+++ b/engines/sci/graphics/screen_item32.h
@@ -61,6 +61,13 @@ private:
 	 */
 	static uint16 _nextObjectId;
 
+	/**
+	 * A serial used to identify the creation order of
+	 * screen items, to ensure a stable sort order for
+	 * screen items with identical priorities and z-indexes.
+	 */
+	static uint32 _nextCreationId;
+
 public:
 	/**
 	 * The parent plane of this screen item.
@@ -120,6 +127,13 @@ private:
 
 public:
 	/**
+	 * The creation order number, which ensures a stable
+	 * sort when screen items with identical priorities and
+	 * z-indexes are added to the screen item list.
+	 */
+	uint32 _creationId;
+
+	/**
 	 * A descriptor for the cel object represented by the
 	 * screen item.
 	 */
@@ -242,7 +256,26 @@ public:
 			}
 
 			if (_position.y + _z == other._position.y + other._z) {
-				return _object < other._object;
+				// SSCI's last resort comparison here is to compare the _object
+				// IDs, but this is wrong and randomly breaks (at least):
+				//
+				// (1) the death dialog at the end of Phant1, where the ID of
+				//     the text is often higher than the ID of the border;
+				// (2) text-based buttons and dialogues in Hoyle5, where the ID
+				//     of the text is often lower than the ID of the
+				//     button/dialogue background.
+				//
+				// This occurs because object IDs (in both ScummVM and SSCI) are
+				// reused, so objects created later may receive a lower ID,
+				// which makes them sort lower when the programmer intended them
+				// to sort higher.
+				//
+				// To fix this problem, we give each ScreenItem a monotonically
+				// increasing insertion ID at construction time, and compare
+				// these insertion IDs instead. They are more stable and cause
+				// objects with identical priority and z-index to be rendered in
+				// the order that they were created.
+				return _creationId < other._creationId;
 			}
 		}
 
@@ -260,7 +293,9 @@ public:
 			}
 
 			if (_position.y + _z == other._position.y + other._z) {
-				return _object > other._object;
+				// This is different than SSCI; see ScreenItem::operator< for an
+				// explanation
+				return _creationId > other._creationId;
 			}
 		}