Don't reset cursor x position during a sim refresh.

The sim may not be complete yet.  And even if it is, the user
didn't ask us to move their cursor.  Just leave it where it is
with an undefined y value.

Fixes https://gitlab.com/kicad/code/kicad/-/issues/15672

eeschema/sim/sim_plot_tab.cpp

@@ -283,17 +283,13 @@ void CURSOR::doSetCoordX( double aValue )
     int minIdx = maxIdx - 1;
 
     // Out of bounds checks
-    if( minIdx < 0 )
+    if( minIdx < 0 || maxIdx >= (int) dataX.size() )
     {
-        minIdx = 0;
-        maxIdx = 1;
-        m_coords.x = dataX[0];
-    }
-    else if( maxIdx >= (int) dataX.size() )
-    {
-        maxIdx = dataX.size() - 1;
-        minIdx = maxIdx - 1;
-        m_coords.x = dataX[maxIdx];
+        // Simulation may not be complete yet, or we may have a cursor off the beginning or end
+        // of the data.  Either way, that's where the user put it.  Don't second guess them; just
+        // leave its y value undefined.
+        m_coords.y = NAN;
+        return;
     }
 
     const double leftX = dataX[minIdx];

eeschema/sim/simulator_frame_ui.cpp

@@ -2438,7 +2438,7 @@ void SIMULATOR_FRAME_UI::updatePlotCursors()
     auto formatValue =
             [this]( double aValue, int aCursorId, int aCol ) -> wxString
             {
-                if( !m_simulatorFrame->SimFinished() && aCol == 1 )
+                if( ( !m_simulatorFrame->SimFinished() && aCol == 1 ) || std::isnan( aValue ) )
                     return wxS( "--" );
                 else
                     return SPICE_VALUE( aValue ).ToString( m_cursorFormats[ aCursorId ][ aCol ] );