Added a bunch of gdk_threads_enter() magic. TBD.

src/control/tools/Selection.cpp

@@ -131,7 +131,9 @@ void RectSelection::paint(cairo_t * cr, GdkRectangle * rect, double zoom) {
 
 	// set the line always the same size on display
 	cairo_set_line_width(cr, 1 / zoom);
+	gdk_threads_enter();
 	gdk_cairo_set_source_color(cr, &selectionColor);
+	gdk_threads_leave();
 
 
 	int aX = MIN(this->sx, this->ex);
@@ -193,7 +195,9 @@ void RegionSelect::paint(cairo_t * cr, GdkRectangle * rect, double zoom) {
 
 		// set the line always the same size on display
 		cairo_set_line_width(cr, 1 / zoom);
+		gdk_threads_enter();
 		gdk_cairo_set_source_color(cr, &selectionColor);
+		gdk_threads_leave();
 
 		RegionPoint * r0 = (RegionPoint *) this->points->data;
 		cairo_move_to(cr, r0->x, r0->y);

src/control/tools/VerticalToolHandler.cpp

@@ -66,6 +66,7 @@ void VerticalToolHandler::paint(cairo_t * cr, GdkRectangle * rect, double zoom)
 
 	cairo_set_line_width(cr, 1);
 
+	gdk_threads_enter();
 	gdk_cairo_set_source_color(cr, &selectionColor);
 
 	double y;
@@ -87,6 +88,7 @@ void VerticalToolHandler::paint(cairo_t * cr, GdkRectangle * rect, double zoom)
 
 	cairo_set_source_surface(cr, this->crBuffer, 0, this->endY * zoom);
 	cairo_paint(cr);
+	gdk_threads_leave();
 }
 
 void VerticalToolHandler::currentPos(double x, double y) {

src/gui/dialog/ImagesDialog.cpp

@@ -131,6 +131,7 @@ private:
 		GtkAllocation alloc;
 		gtk_widget_get_allocation(this->widget, &alloc);
 
+		gdk_threads_enter();
 		if (this->crBuffer == NULL) {
 			this->crBuffer = cairo_image_surface_create(CAIRO_FORMAT_ARGB32, alloc.width, alloc.height);
 
@@ -191,6 +192,7 @@ private:
 
 		cairo_paint(cr);
 		cairo_destroy(cr);
+		gdk_threads_leave();
 	}
 
 public:

src/gui/sidebar/previews/SidebarPreviewPage.cpp

@@ -90,8 +90,10 @@ void SidebarPreviewPage::paint() {
 		}
 
 		this->firstPainted = true;
+		gdk_threads_enter();
 		gdk_window_set_background(widget->window, &widget->style->white);
 		gtk_widget_queue_draw(this->widget);
+		gdk_threads_leave();
 		return;
 	}
 
@@ -128,6 +130,7 @@ void SidebarPreviewPage::paint() {
 		repaint();
 	}
 
+	gdk_threads_enter();
 	cairo_t * cr = gdk_cairo_create(widget->window);
 	cairo_set_source_surface(cr, this->crBuffer, 0, 0);
 	cairo_paint(cr);
@@ -154,6 +157,7 @@ void SidebarPreviewPage::paint() {
 	}
 
 	cairo_destroy(cr);
+	gdk_threads_leave();
 
 	g_mutex_unlock(this->drawingMutex);
 }

src/gui/toolbarMenubar/ToolZoomSlider.cpp

@@ -60,9 +60,11 @@ void ToolZoomSlider::updateScaleMarks() {
 		return;
 	}
 
+	gdk_threads_enter();
 	gtk_scale_clear_marks( GTK_SCALE(this->slider));
 	gtk_scale_add_mark(GTK_SCALE(this->slider), zoom->getZoom100(), horizontal ? GTK_POS_BOTTOM : GTK_POS_RIGHT, NULL);
 	gtk_scale_add_mark(GTK_SCALE(this->slider), zoom->getZoomFit(), horizontal ? GTK_POS_BOTTOM : GTK_POS_RIGHT, NULL);
+	gdk_threads_leave();
 }
 
 void ToolZoomSlider::setHorizontal(bool horizontal) {

src/gui/widgets/SelectColor.cpp

@@ -147,6 +147,7 @@ static gboolean selectcolor_expose(GtkWidget *widget, GdkEventExpose *event) {
 
 static void selectcolor_paint(GtkWidget *widget) {
 	cairo_t *cr;
+	gdk_threads_enter();
 	cr = gdk_cairo_create(widget->window);
 	cairo_fill(cr);
 
@@ -203,6 +204,7 @@ static void selectcolor_paint(GtkWidget *widget) {
 	cairo_stroke(cr);
 
 	cairo_destroy(cr);
+	gdk_threads_leave();
 }
 
 static void selectcolor_destroy(GtkObject *object) {

src/gui/widgets/XournalWidget.cpp

@@ -617,11 +617,13 @@ cairo_t * gtk_xournal_create_cairo_for(GtkWidget * widget, PageView * view) {
 	double zoom = xournal->view->getZoom();
 
 	// TODO LOW PRIO: stroke draw to this cairo surface look a little different than rendererd to a cairo surface
+	gdk_threads_enter();
 	cairo_t * cr = gdk_cairo_create(GTK_WIDGET(widget)->window);
 	int x = view->getX() - xournal->x;
 	int y = view->getY() - xournal->y;
 	cairo_translate(cr, x, y);
 	cairo_scale(cr, zoom, zoom);
+	gdk_threads_leave();
 
 	return cr;
 }
@@ -658,6 +660,7 @@ static gboolean gtk_xournal_expose(GtkWidget * widget, GdkEventExpose * event) {
 
 	GtkXournal * xournal = GTK_XOURNAL(widget);
 
+	gdk_threads_enter();
 	cairo_t * cr = gdk_cairo_create(GTK_WIDGET(widget)->window);
 
 	ArrayIterator<PageView *> it = xournal->view->pageViewIterator();
@@ -744,6 +747,7 @@ static gboolean gtk_xournal_expose(GtkWidget * widget, GdkEventExpose * event) {
 	}
 
 	cairo_destroy(cr);
+	gdk_threads_leave();
 
 	return true;
 }

src/view/DocumentView.cpp

@@ -261,6 +261,7 @@ void DocumentView::paintBackgroundImage() {
 		cairo_matrix_t matrix = { 0 };
 		cairo_get_matrix(cr, &matrix);
 
+		gdk_threads_enter();
 		int width = gdk_pixbuf_get_width(pixbuff);
 		int height = gdk_pixbuf_get_height(pixbuff);
 
@@ -273,6 +274,7 @@ void DocumentView::paintBackgroundImage() {
 		cairo_paint(cr);
 
 		cairo_set_matrix(cr, &matrix);
+		gdk_threads_leave();
 	}
 }