Fix #31176 - Appearing and disappearing courtesy clef

When courtesy clefs are turned off, some sequences of steps involving clef changes across system breaks result in spurious courtesy clefs being shown. Also, occasionally, the layout oscillates between showing / hiding courtesy clefs and/or fitting its measure in the same system or the next system. See http://musescore.org/en/node/31176 for details and examples.

libmscore/clef.cpp

@@ -21,6 +21,7 @@
 #include "xml.h"
 #include "sym.h"
 #include "symbol.h"
+#include "system.h"
 #include "score.h"
 #include "staff.h"
 #include "segment.h"
@@ -176,24 +177,16 @@ void Clef::layout()
             bool showClef = true;
             // only if there is a clef change
             if (!bHide && tick > 0 && stf->clef(tick) != stf->clef(tick-1)) {
-                  // locate clef at the begining of next measure, if any
-                  Clef*       clefNext    = nullptr;
-                  Segment*    clefSegNext = nullptr;
                   Measure*    meas        = static_cast<Measure*>(clefSeg->parent());
-                  Measure*    measNext    = meas->nextMeasure();
-                  if (measNext) {
-                        clefSegNext = measNext->findSegment(Segment::Type::Clef, tick);
-                        if (clefSegNext)
-                              clefNext = static_cast<Clef*>(clefSegNext->element(track()));
-                        }
-                  // show this clef if: it is not a courtesy clef (no next clef or not at the end of the measure)
-                  showClef = !clefNext || (clefSeg->tick() != meas->tick() + meas->ticks())
+                  showClef =                    // show this clef if:
+                        // it is not a courtesy clef (not at the end of the last measure of the system)
+                        (meas != meas->system()->lastMeasure()) || (clefSeg->tick() != meas->tick() + meas->ticks())
                         // if courtesy clef: show if score has courtesy clefs on
                         || ( score()->styleB(StyleIdx::genCourtesyClef)
-                        // AND measure is not at the end of a repeat or of a section
-                        && !( (meas->repeatFlags() & Repeat::END) || meas->sectionBreak() )
-                        // AND this clef has courtesy clef turned on
-                        && showCourtesy() );
+                              // AND measure is not at the end of a repeat or of a section
+                              && !( (meas->repeatFlags() & Repeat::END) || meas->sectionBreak() )
+                              // AND this clef has courtesy clef turned on
+                              && showCourtesy() );
                   bHide |= !showClef;
                   }