Fix hidden staff conditions with spanners

[miiizen]

Resolves: #20782

Spanners should only make the following bar visible if the spanner is a slur.

[Jojo-Schmitz]

While at it, couldn't you also change this a few lines above?

diff --git a/src/engraving/rendering/dev/systemlayout.cpp b/src/engraving/rendering/dev/systemlayout.cpp
index d8833d5e31..a6e3191af8 100644
--- a/src/engraving/rendering/dev/systemlayout.cpp
+++ b/src/engraving/rendering/dev/systemlayout.cpp
@@ -596,7 +596,7 @@ void SystemLayout::hideEmptyStaves(System* system, LayoutContext& ctx, bool isFi

     Fraction stick = system->measures().front()->tick();
     Fraction etick = system->measures().back()->endTick();
-    auto spanners = ctx.dom().spannerMap().findOverlapping(stick.ticks(), etick.ticks() - 1);
+    auto& spanners = ctx.dom().spannerMap().findOverlapping(stick.ticks(), etick.ticks() - 1);

     for (const Staff* staff : ctx.dom().staves()) {
         SysStaff* ss  = system->staff(staffIdx);
@@ -609,7 +609,7 @@ void SystemLayout::hideEmptyStaves(System* system, LayoutContext& ctx, bool isFi
                 && !(isFirstSystem && ctx.conf().styleB(Sid::dontHideStavesInFirstSystem))
                 && hideMode != Staff::HideMode::NEVER)) {
             bool hideStaff = true;
-            for (auto spanner : spanners) {
+            for (auto& spanner : spanners) {
                 if (spanner.value->staff() == staff
                     && !spanner.value->systemFlag()
                     && !spanner.value->isHairpin()

It'd fix code warnings "'auto' doesn't deduce a reference . A possible unintended copy is being made"

Also why only in ...dev/systemlayout.cpp and not also in ...stable/systemlayout.cpp?

[Jojo-Schmitz]

This additional test

renders the previous one

useless, as not that staff isn't empty anymore

[Jojo-Schmitz]

Can't that hairpin condition get dropped here?
Looking at the reasoning for having added it in #19679, which I meanwhile believe to have been wrong

[RomanPudashkin]

const auto& or const SpannerMap::IntervalList&

[Jojo-Schmitz]

Ah, sorry for having misled you

[Jojo-Schmitz]

Hmm, MSVC believes it to be const without explicitly being qualified as such