At least with a recent Linux ASan+UBSan master build, with Contoso-Presentation.pptx as obtained from bug 127258 comment 0, "Slide Show - Start from First Slide" and then pressing space bar four times to go past the first effect on the third slide, sometimes causes

[...]
> debug:16922:16922: SBSBSB.1 -0.757713 0.5 -1.51543
> debug:16922:16922: SBSBSB.2 1 1
> debug:16922:16922: SBSBSB.8 -0.515426 -1
> debug:16922:16922: SBSBSB.10 -1
> slideshow/source/engine/activities/simplecontinuousactivitybase.cxx:241:53: runtime error: -1 is outside the range of representable values of type 'unsigned int'
>  #0 in slideshow::internal::SimpleContinuousActivityBase::perform() at slideshow/source/engine/activities/simplecontinuousactivitybase.cxx:241:53 (instdir/program/../program/libslideshowlo.so +0x134b4b2)
>  #1 in slideshow::internal::ActivitiesQueue::process() at slideshow/source/engine/activitiesqueue.cxx:103:44 (instdir/program/../program/libslideshowlo.so +0x133aadc)
>  #2 in (anonymous namespace)::SlideShowImpl::update(double&) at slideshow/source/engine/slideshowimpl.cxx:1977:31 (instdir/program/../program/libslideshowlo.so +0x18550e5)
>  #3 in sd::SlideshowImpl::updateSlideShow() at sd/source/ui/slideshow/slideshowimpl.cxx:1673:21 (instdir/program/../program/libsdlo.so +0x4a14de8)
>  #4 in Scheduler::ProcessTaskScheduling() at vcl/source/app/scheduler.cxx:479:20 (instdir/program/libvcllo.so +0x7b5ab95)
>  #5 in sal_gtk_timeout_dispatch(_GSource*, int (*)(void*), void*) at vcl/unx/gtk3/gtk3gtkdata.cxx:761:45 (instdir/program/libvclplug_gtk3lo.so +0xd60d41)
>  #6 in g_main_dispatch at ../glib/gmain.c:3189:28 (/lib64/libglib-2.0.so.0 +0x4fedc)
>  #7 in g_main_context_dispatch at ../glib/gmain.c:3854:7 (/lib64/libglib-2.0.so.0 +0x4fedc)
>  #8 in g_main_context_iterate at ../glib/gmain.c:3927:5 (/lib64/libglib-2.0.so.0 +0x5026f)
>  #9 in g_main_context_iteration at ../glib/gmain.c:3988:12 (/lib64/libglib-2.0.so.0 +0x50312)
>  #10 in GtkSalData::Yield(bool, bool) at vcl/unx/gtk3/gtk3gtkdata.cxx:528:31 (instdir/program/libvclplug_gtk3lo.so +0xd5bb41)
>  #11 in ImplYield(bool, bool) at vcl/source/app/svapp.cxx:447:48 (instdir/program/libvcllo.so +0x7be6b9a)
>  #12 in Application::Yield() at vcl/source/app/svapp.cxx:511:5 (instdir/program/libvcllo.so +0x7be60e9)
>  #13 in Application::Execute() at vcl/source/app/svapp.cxx:428:9 (instdir/program/libvcllo.so +0x7be60e9)
>  #14 in desktop::Desktop::Main() at desktop/source/app/app.cxx:1620:17 (instdir/program/libsofficeapp.so +0x732047)
>  #15 in ImplSVMain() at vcl/source/app/svmain.cxx:191:35 (instdir/program/libvcllo.so +0x7c40097)
>  #16 in soffice_main at desktop/source/app/sofficemain.cxx:177:12 (instdir/program/libsofficeapp.so +0x83c920)
>  #17 in sal_main at desktop/source/app/main.c:48:15 (instdir/program/soffice.bin +0x32502a)
>  #18 in main at desktop/source/app/main.c:47:1 (instdir/program/soffice.bin +0x32502a)
>  #19 in __libc_start_main at /usr/src/debug/glibc-2.29-24-g2ec0b166bf/csu/../csu/libc-start.c:308:16 (/lib64/libc.so.6 +0x23f32)
>  #20 in _start at <null> (instdir/program/soffice.bin +0x24e02d)
> 
> SUMMARY: UndefinedBehaviorSanitizer: float-cast-overflow slideshow/source/engine/activities/simplecontinuousactivitybase.cxx:241:53 in 

(with "SBSBSB..." SAL_DEBUG output produced via the patch given below).

It looks like a negative nT leading to a negative nRepeats is not actually expected here?  (Where the negative maTimer.getElapsedTime() is presumably caused by calls to

>                 mpTimer->adjustTimer( -fLag );

in ActivitiesQueue::process, slideshow/source/engine/activitiesqueue.cxx.)

The patch producing the "SBSBSB..." SAL_DEBUG output is:

> diff --git a/slideshow/source/engine/activities/simplecontinuousactivitybase.cxx b/slideshow/source/engine/activities/simplecontinuousactivitybase.cxx
> index 5566b5883f78..1f91b341cb97 100644
> --- a/slideshow/source/engine/activities/simplecontinuousactivitybase.cxx
> +++ b/slideshow/source/engine/activities/simplecontinuousactivitybase.cxx
> @@ -118,6 +118,7 @@ namespace slideshow
>  
>              const double nCurrElapsedTime( maTimer.getElapsedTime() );
>              double nT( nCurrElapsedTime / mnMinSimpleDuration );
> +            SAL_DEBUG("SBSBSB.1 "<<nCurrElapsedTime<<" "<<mnMinSimpleDuration<<" "<<nT);
>  
>  
>              // one of the stop criteria reached?
> @@ -138,6 +139,7 @@ namespace slideshow
>                  const double nEffectiveRepeat( isAutoReverse() ?
>                                                 2.0*nRepeatCount :
>                                                 nRepeatCount );
> +                SAL_DEBUG("SBSBSB.2 "<<nRepeatCount<<" "<<nEffectiveRepeat);
>  
>                  // time (or frame count) elapsed?
>                  if( nEffectiveRepeat <= nT )
> @@ -149,6 +151,7 @@ namespace slideshow
>  
>                      // clamp animation to max permissible value
>                      nT = nEffectiveRepeat;
> +                    SAL_DEBUG("SBSBSB.3 "<<nT);
>                  }
>              }
>  
> @@ -165,6 +168,7 @@ namespace slideshow
>                  // divert active duration into repeat and
>                  // fractional part.
>                  const double nFractionalActiveDuration( modf(nT, &nRepeats) );
> +                SAL_DEBUG("SBSBSB.4 "<<nFractionalActiveDuration<<" "<<nRepeats);
>  
>                  // for auto-reverse, map ranges [1,2), [3,4), ...
>                  // to ranges [0,1), [1,2), etc.
> @@ -172,17 +176,20 @@ namespace slideshow
>                  {
>                      // we're in an odd range, reverse sweep
>                      nRelativeSimpleTime = 1.0 - nFractionalActiveDuration;
> +                    SAL_DEBUG("SBSBSB.5 "<<nRelativeSimpleTime);
>                  }
>                  else
>                  {
>                      // we're in an even range, pass on as is
>                      nRelativeSimpleTime = nFractionalActiveDuration;
> +                    SAL_DEBUG("SBSBSB.6 "<<nRelativeSimpleTime);
>                  }
>  
>                  // effective repeat count for autoreverse is half of
>                  // the input time's value (each run of an autoreverse
>                  // cycle is half of a repeat)
>                  nRepeats /= 2;
> +                SAL_DEBUG("SBSBSB.7 "<<nRepeats);
>              }
>              else
>              {
> @@ -194,6 +201,7 @@ namespace slideshow
>                  // nT gives the relative simple time, and the
>                  // integer part the number of full repeats:
>                  nRelativeSimpleTime = modf(nT, &nRepeats);
> +                SAL_DEBUG("SBSBSB.8 "<<nRelativeSimpleTime<<" "<<nRepeats);
>  
>                  // clamp repeats to max permissible value (maRepeats.getValue() - 1.0)
>                  if( isRepeatCountValid() &&
> @@ -220,12 +228,14 @@ namespace slideshow
>                      // nRelativeSimpleTime=1.0.
>                      nRelativeSimpleTime = 1.0;
>                      nRepeats -= 1.0;
> +                    SAL_DEBUG("SBSBSB.9 "<<nRelativeSimpleTime<<" "<<nRepeats);
>                  }
>              }
>  
>              // actually perform something
>              // ==========================
>  
> +            SAL_DEBUG("SBSBSB.10 "<<nRepeats);
>              simplePerform( nRelativeSimpleTime,
>                             // nRepeats is already integer-valued
>                             static_cast<sal_uInt32>( nRepeats ) );