diff --git a/gtk2_ardour/ghostregion.h b/gtk2_ardour/ghostregion.h
index bf516cae30..6c869f49e0 100644
--- a/gtk2_ardour/ghostregion.h
+++ b/gtk2_ardour/ghostregion.h
@@ -28,6 +28,7 @@
 #include <vector>
 #include <boost/unordered_map.hpp>
 
+#include "evoral/Note.h"
 #include "pbd/signals.h"
 
 #include "gtkmm2ext/colors.h"
diff --git a/gtk2_ardour/insert_remove_time_dialog.h b/gtk2_ardour/insert_remove_time_dialog.h
index dfb88dd0eb..f20cc2ebd5 100644
--- a/gtk2_ardour/insert_remove_time_dialog.h
+++ b/gtk2_ardour/insert_remove_time_dialog.h
@@ -20,6 +20,7 @@
 #include "public_editor.h"
 #include "editing.h"
 #include "audio_clock.h"
+#include <gtkmm/comboboxtext.h>
 
 class InsertRemoveTimeDialog : public ArdourDialog
 {
diff --git a/gtk2_ardour/note_base.cc b/gtk2_ardour/note_base.cc
index a6ad6e6bc9..bf75c1a4a7 100644
--- a/gtk2_ardour/note_base.cc
+++ b/gtk2_ardour/note_base.cc
@@ -29,10 +29,14 @@
 
 #include "note_base.h"
 #include "public_editor.h"
-#include "editing_syms.h"
 #include "keyboard.h"
 #include "midi_region_view.h"
 
+/* clang-format off */
+// Include last, when GRIDTYPE has been defined by editing.h via midi_region_view.h
+#include "editing_syms.h"
+/* clang-format on */
+
 using namespace std;
 using namespace Gtkmm2ext;
 using ARDOUR::MidiModel;
diff --git a/gtk2_ardour/playlist_selection.h b/gtk2_ardour/playlist_selection.h
index fa351d8a3d..540e2f93a2 100644
--- a/gtk2_ardour/playlist_selection.h
+++ b/gtk2_ardour/playlist_selection.h
@@ -20,13 +20,11 @@
 #ifndef __ardour_gtk_playlist_selection_h__
 #define __ardour_gtk_playlist_selection_h__
 
+#include "ardour/playlist.h"
+
 #include <list>
 #include <memory>
 
-namespace ARDOUR {
-	class Playlist;
-}
-
 struct PlaylistSelection : std::list<std::shared_ptr<ARDOUR::Playlist> > {
 public:
 	const_iterator
diff --git a/gtk2_ardour/port_group.h b/gtk2_ardour/port_group.h
index 79856b071e..74767144d4 100644
--- a/gtk2_ardour/port_group.h
+++ b/gtk2_ardour/port_group.h
@@ -31,12 +31,12 @@
 #include <gtkmm/widget.h>
 #include <gtkmm/checkbutton.h>
 
+#include "ardour/bundle.h"
 #include "ardour/data_type.h"
 #include "ardour/types.h"
 
 namespace ARDOUR {
 	class Session;
-	class Bundle;
 	class Processor;
 	class IO;
 }
diff --git a/gtk2_ardour/port_matrix_grid.h b/gtk2_ardour/port_matrix_grid.h
index afdaf5607b..21f98e2e91 100644
--- a/gtk2_ardour/port_matrix_grid.h
+++ b/gtk2_ardour/port_matrix_grid.h
@@ -31,10 +31,6 @@
 class PortMatrix;
 class PortMatrixBody;
 
-namespace ARDOUR {
-	class Bundle;
-}
-
 /**  The grid part of the port matrix */
 class PortMatrixGrid : public PortMatrixComponent
 {
diff --git a/gtk2_ardour/sys_ex.h b/gtk2_ardour/sys_ex.h
index 48ac5cced2..2dea6110bd 100644
--- a/gtk2_ardour/sys_ex.h
+++ b/gtk2_ardour/sys_ex.h
@@ -20,12 +20,9 @@
 #ifndef __SYSEX_H__
 #define __SYSEX_H__
 
+#include "canvas/flag.h"
 #include "midi_region_view.h"
 
-namespace ArdourCanvas {
-	class Flag;
-}
-
 class SysEx
 {
 public:
diff --git a/libs/ardour/ardour/operations.h b/libs/ardour/ardour/operations.h
index 81a392ae8a..a661fbfa76 100644
--- a/libs/ardour/ardour/operations.h
+++ b/libs/ardour/ardour/operations.h
@@ -18,6 +18,7 @@
  */
 
 #include "ardour/libardour_visibility.h"
+#include "pbd/properties.h"
 
 /** These are GQuarks for a subset of UI operations.  We use these
  *  so that the undo system can be queried to find out what operations
diff --git a/libs/ardour/ardour/route_group_member.h b/libs/ardour/ardour/route_group_member.h
index b3023fe5cf..73f303b30a 100644
--- a/libs/ardour/ardour/route_group_member.h
+++ b/libs/ardour/ardour/route_group_member.h
@@ -21,6 +21,7 @@
 #ifndef __libardour_route_group_member_h__
 #define __libardour_route_group_member_h__
 
+#include "ardour/libardour_visibility.h"
 #include "pbd/controllable.h"
 #include "pbd/signals.h"
 
diff --git a/libs/ardour/ardour/session_configuration.h b/libs/ardour/ardour/session_configuration.h
index 8bcd053c1b..f100ca4667 100644
--- a/libs/ardour/ardour/session_configuration.h
+++ b/libs/ardour/ardour/session_configuration.h
@@ -24,6 +24,7 @@
 #include <map>
 #include <string>
 
+#include "ardour/types.h"
 #include "pbd/configuration.h"
 
 namespace ARDOUR {
diff --git a/libs/gtkmm2ext/gtkmm2ext/persistent_tooltip.h b/libs/gtkmm2ext/gtkmm2ext/persistent_tooltip.h
index b51a2c50fa..434ace298e 100644
--- a/libs/gtkmm2ext/gtkmm2ext/persistent_tooltip.h
+++ b/libs/gtkmm2ext/gtkmm2ext/persistent_tooltip.h
@@ -22,6 +22,7 @@
 
 #include <sigc++/trackable.h>
 
+#include <gtkmm.h>
 #include "gtkmm2ext/visibility.h"
 
 namespace Gtkmm2ext {
diff --git a/libs/surfaces/us2400/control_group.h b/libs/surfaces/us2400/control_group.h
index debfe26b0f..40c1a2d0a4 100644
--- a/libs/surfaces/us2400/control_group.h
+++ b/libs/surfaces/us2400/control_group.h
@@ -19,6 +19,7 @@
 #ifndef __ardour_us2400_control_protocol_control_group_h__
 #define __ardour_us2400_control_protocol_control_group_h__
 
+#include <string>
 #include <vector>
 
 namespace ArdourSurface {