Libedit: transfer current screen settings on part load

When loading a part in libedit, also transfer certain screen
settings to the incoming SCH_SCREEN, from the current
SCH_SCREEN.

Currently, the grid settings are transferred, but this could
also do other settings if needed.

Also remove some duplicate commentary and mention why the
grid is set in a SCH_SCREEN even though the BASE_SCREEN ctor
sets the same default.

Fixes: lp:1815108
* https://bugs.launchpad.net/kicad/+bug/1815108

common/base_screen.cpp

@@ -174,11 +174,7 @@ bool BASE_SCREEN::SetPreviousZoom()
     return false;
 }
 
-/* Build the list of human readable grid list.
- * The list shows the grid size both in mils or mm.
- * aMmFirst = true to have mm first and mils after
- *            false to have mils first and mm after
- */
+
 int BASE_SCREEN::BuildGridsChoiceList( wxArrayString& aGridsList, bool aMmFirst) const
 {
     wxString msg;

eeschema/libedit/libedit.cpp

@@ -151,6 +151,22 @@ bool LIB_EDIT_FRAME::LoadComponentFromCurrentLib( const wxString& aAliasName, in
     return true;
 }
 
+/**
+ * Synchronise screen settings from a current screen into another screen.
+ *
+ * This can be used, for example, when loading a new screen into a frame,
+ * but you want the new screen to inherit some settings (e.g. grids) from the
+ * frame's current screen.
+ *
+ * @param aCurrentScreen    the existing frame screen
+ * @param aIncomingScreen   a screen that is intended to replace the current screen
+ */
+static void synchronizeLibEditScreenSettings(
+        const SCH_SCREEN& aCurrentScreen, SCH_SCREEN& aIncomingScreen )
+{
+    aIncomingScreen.SetGrid( aCurrentScreen.GetGridSize() );
+}
+
 
 bool LIB_EDIT_FRAME::LoadOneLibraryPartAux( LIB_ALIAS* aEntry, const wxString& aLibrary,
                                             int aUnit, int aConvert )
@@ -172,8 +188,18 @@ bool LIB_EDIT_FRAME::LoadOneLibraryPartAux( LIB_ALIAS* aEntry, const wxString& a
     m_unit = aUnit > 0 ? aUnit : 1;
     m_convert = aConvert > 0 ? aConvert : 1;
 
-    auto s = m_libMgr->GetScreen( lib_part->GetName(), aLibrary );
-    SetScreen( s );
+    // The buffered screen for the part
+    SCH_SCREEN* part_screen = m_libMgr->GetScreen( lib_part->GetName(), aLibrary );
+
+    const SCH_SCREEN* curr_screen = GetScreen();
+
+    // Before we set the frame screen, transfer any settings from the current
+    // screen that we want to keep to the incoming (buffered) part's screen
+    // which could be out of date relative to the current screen.
+    if( curr_screen )
+        synchronizeLibEditScreenSettings( *curr_screen, *part_screen );
+
+    SetScreen( part_screen );
     SetCurPart( new LIB_PART( *lib_part ) );
     SetCurLib( aLibrary );
 

eeschema/sch_screen.cpp

@@ -105,7 +105,9 @@ SCH_SCREEN::SCH_SCREEN( KIWAY* aKiway ) :
     for( unsigned i = 0; i < arrayDim( SchematicGridList ); i++ )
         AddGrid( SchematicGridList[i] );
 
-    SetGrid( wxRealPoint( 50, 50 ) );   // Default grid size.
+    // Set the default grid size, now that the grid list is populated
+    SetGrid( wxRealPoint( 50, 50 ) );
+
     m_refCount = 0;
 
     // Suitable for schematic only. For libedit and viewlib, must be set to true