From 865863ab6fb2d8cd464bc6637955eaf11bb41b57 Mon Sep 17 00:00:00 2001
From: Christoph Oelckers <c.oelckers@zdoom.fake>
Date: Sun, 2 Oct 2016 14:37:26 +0200
Subject: [PATCH] - fixed: AFastProjectile did not work properly for perfectly
 vertically moving missiles. Like AActor::Tick it needs to ensure that a tiny
 bit of lateral movement is present so that collision detection and the
 Effect() function work as intended.

---
 src/g_shared/a_fastprojectile.cpp | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/src/g_shared/a_fastprojectile.cpp b/src/g_shared/a_fastprojectile.cpp
index c1253f966..ec736bfea 100644
--- a/src/g_shared/a_fastprojectile.cpp
+++ b/src/g_shared/a_fastprojectile.cpp
@@ -54,6 +54,12 @@ void AFastProjectile::Tick ()
 	// Handle movement
 	if (!Vel.isZero() || (Z() != floorz))
 	{
+		// force some lateral movement so that collision detection works as intended.
+		if ((flags & MF_MISSILE) && Vel.X == 0 && Vel.Y == 0 && !IsZeroDamage())
+		{
+			Vel.X = MinVel;
+		}
+
 		frac = Vel / count;
 		changexy = frac.X != 0 || frac.Y != 0;
 		int ripcount = count / 8;