SCI32: Detect and handle tight loops around kGetEvent

In SSCI, mouse events are received through a hardware interrupt
and the cursor is drawn directly to the graphics card by the
interrupt handler. This allows game scripts to omit calls to
kFrameOut without stopping the mouse cursor from being redrawn
in response to mouse movement.

ScummVM, in contrast, needs to poll for events and submit screen
updates explicitly from the main thread. Submitting screen updates
may block on vsync, which means that this call should really only
be made once per frame, just after the game has finished updating
its back buffer. The closest signal in SCI32 for having completed
drawing a frame is the kFrameOut call, so this is where the
update is submitted (by calling OSystem::updateScreen).

The problem with the approach in ScummVM is that, even though the
mouse position is being updated (by calls to kGetEvent) and drawn
to the backend's back buffer (by GfxCursor32::drawToHardware),
OSystem::updateScreen is never called during game loops that omit
calls to kFrameOut.

This commit introduces a workaround that looks at the number of
times kGetEvent is called between calls to kFrameOut. If the
number of kGetEvent calls is higher than usual (where "usual"
seems to be 0 or 1), we assume that the game is running one of
these tight event loops, and kGetEvent starts calling
OSystem::updateScreen until the next kFrameOut call. We also
then start throttling the calls to kGetEvent to keep CPU usage
down. This fixes at least two such known loops:

1. When interacting with the menu bar at the top of the screen
   in LSL6hires;
2. When restoring a game in Phant2 and sitting on the "click mouse"
   screen.

A similar workaround may also be needed for kGetTime, though loops
around kGetTime should preferably be replaced using a script patch
to call kWait instead.

1 files changed, 3 insertions, 1 deletions

diff --git a/engines/sci/engine/state.h b/engines/sci/engine/state.h
index c41f86c781..57839ab04e 100644
--- a/engines/sci/engine/state.h
+++ b/engines/sci/engine/state.h
@@ -128,7 +128,9 @@ public:
 	void speedThrottler(uint32 neededSleep);
 	void wait(int16 ticks);
 
-	uint32 _throttleCounter; /**< total times kAnimate was invoked */
+#ifdef ENABLE_SCI32
+	uint32 _eventCounter; /**< total times kGetEvent was invoked since the last call to kFrameOut */
+#endif
 	uint32 _throttleLastTime; /**< last time kAnimate was invoked */
 	bool _throttleTrigger;
 	bool _gameIsBenchmarking;