Cautions re. size of Preference pages

Clarify meaning / purpose of assert in PrefsDialog
and add scrollers to pages that may grow in future versions.

src/prefs/EffectsPrefs.cpp

@@ -57,6 +57,7 @@ void EffectsPrefs::PopulateOrExchange(ShuttleGui & S)
 {
    S.SetBorder(2);
 
+   S.StartScroller();
    S.StartStatic(_("Enable Effects"));
    {
 
@@ -158,6 +159,7 @@ void EffectsPrefs::PopulateOrExchange(ShuttleGui & S)
                     true);
    }
    S.EndStatic();
+   S.EndScroller();
 #endif
 }
 

src/prefs/GUIPrefs.cpp

@@ -124,6 +124,7 @@ void GUIPrefs::PopulateOrExchange(ShuttleGui & S)
 {
    S.SetBorder(2);
 
+   S.StartScroller();
    S.StartStatic(_("Display"));
    {
       S.StartMultiColumn(2);
@@ -206,6 +207,7 @@ void GUIPrefs::PopulateOrExchange(ShuttleGui & S)
                     true);
    }
    S.EndStatic();
+   S.EndScroller();
 }
 
 bool GUIPrefs::Commit()

src/prefs/PrefsDialog.cpp

@@ -332,11 +332,13 @@ PrefsDialog::PrefsDialog
          mCategories->ExpandNode(iPage, it->expanded);
    }
 
-   // This ASSERT used to limit us to 800 x 600.
+   // This ASSERT was originally used to limit us to 800 x 600.
-   // However, we think screens have got bigger now, and that was a bit too restrictive.
+   // However, the range of screen sizes and dpi of modern (2018) displays
-   // The impetus for increasing the limit (before we ASSERT) was that this ASSERT
+   // makes pixel dimensions an inadequate measure of usability, so
-   // was firing with wxWidgets 3.0, which has slightly different sizer behaviour.
+   // now we only ASSERT that preferences will fit in the client display
-   // Now takes display size into account to counter firing with high dpi displays.
+   // rectangle of the developer / tester's monitor.
+   // Use scrollers when necessary to ensure that preference pages will
+   // be fully visible.
    wxRect screenRect(wxGetClientDisplayRect());
    wxASSERT_MSG(sz.x <= screenRect.width && sz.y <= screenRect.height, wxT("Preferences dialog exceeds max size"));
 

src/prefs/TracksBehaviorsPrefs.cpp

@@ -60,6 +60,7 @@ void TracksBehaviorsPrefs::PopulateOrExchange(ShuttleGui & S)
 {
    S.SetBorder(2);
 
+   S.StartScroller();
    S.StartStatic(_("Behaviors"));
    {
       S.TieCheckBox(_("A&uto-select, if selection required"),
@@ -104,6 +105,7 @@ void TracksBehaviorsPrefs::PopulateOrExchange(ShuttleGui & S)
       S.EndMultiColumn();
    }
    S.EndStatic();
+   S.EndScroller();
 }
 
 bool TracksBehaviorsPrefs::Commit()

src/prefs/TracksPrefs.cpp

@@ -120,6 +120,7 @@ void TracksPrefs::PopulateOrExchange(ShuttleGui & S)
 {
    S.SetBorder(2);
 
+   S.StartScroller();
    S.StartStatic(_("Display"));
    {
       S.TieCheckBox(_("&Pinned Recording/Playback head"),
@@ -183,6 +184,7 @@ void TracksPrefs::PopulateOrExchange(ShuttleGui & S)
       }
    }
    S.EndStatic();
+   S.EndScroller();
 }
 
 bool TracksPrefs::GetPinnedHeadPreference()