Forward-port upstream patch submission
https://sourceforge.net/p/xournal/patches/58/
https://sourceforge.net/p/xournal/patches/_discuss/thread/f1d7a1d7/4c10/attachment/xournal-0.4.5-dawb-aspectratio.patch

It applies cleanly, just to xo-selection.c instead of xo-paint.c.

Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>

diff -Narpu --exclude '*.orig' --exclude '*.rej' --exclude '*~' xournal-0.4.8.orig/src/xo-selection.c xournal-0.4.8/src/xo-selection.c
--- xournal-0.4.8.orig/src/xo-selection.c	2014-06-06 16:41:45.000000000 -0700
+++ xournal-0.4.8/src/xo-selection.c	2017-02-09 15:31:08.960817336 -0800
@@ -347,6 +347,12 @@ gboolean start_resizesel(GdkEvent *event
     ui.selection->new_x2 = ui.selection->bbox.right;
     gnome_canvas_item_set(ui.selection->canvas_item, "dash", NULL, NULL);
     update_cursor_for_resize(pt);
+
+    // Check whether we should preserve the aspect ratio
+    if (event->button.button == 3)
+    	ui.cur_brush->tool_options |= TOOLOPT_SELECT_PRESERVE;
+    else
+    	ui.cur_brush->tool_options &= ~TOOLOPT_SELECT_PRESERVE;
     return TRUE;
   }
   return FALSE;
@@ -498,6 +504,38 @@ void continue_resizesel(GdkEvent *event)
   if (ui.selection->resizing_left) ui.selection->new_x1 = pt[0];
   if (ui.selection->resizing_right) ui.selection->new_x2 = pt[0];
 
+  if (ui.cur_brush->tool_options & TOOLOPT_SELECT_PRESERVE) {
+	  double aspectratio = (ui.selection->bbox.top - ui.selection->bbox.bottom)/(ui.selection->bbox.right - ui.selection->bbox.left);
+	  double newheight = ui.selection->new_y1 - ui.selection->new_y2;
+	  double newwidth = ui.selection->new_x2 - ui.selection->new_x1;
+	  gboolean boundheight;
+
+	  // Resizing from top or bottom only
+	  if ((ui.selection->resizing_top || ui.selection->resizing_bottom) && !(ui.selection->resizing_left || ui.selection->resizing_right))
+		  boundheight = 0;
+	  // Resizing from right or left only
+	  else if (!(ui.selection->resizing_top || ui.selection->resizing_bottom) && (ui.selection->resizing_left || ui.selection->resizing_right))
+		  boundheight = 1;
+	  // Resizing from a corner
+	  else if (newheight/aspectratio > newwidth)
+		  boundheight = 0;
+	  else
+		  boundheight = 1;
+
+	  if (boundheight) {
+		  // Bound the height
+		  newheight = newwidth*aspectratio;
+		  if (ui.selection->resizing_top) ui.selection->new_y1 = ui.selection->new_y2 + newheight;
+		  else ui.selection->new_y2 = ui.selection->new_y1 - newheight;
+	  }
+	  else {
+		  // Bound the width
+		  newwidth = newheight/aspectratio;
+		  if (ui.selection->resizing_left) ui.selection->new_x1 = ui.selection->new_x2 - newwidth;
+		  else ui.selection->new_x2 = ui.selection->new_x1 + newwidth;
+	  }
+  }
+
   gnome_canvas_item_set(ui.selection->canvas_item, 
     "x1", ui.selection->new_x1, "x2", ui.selection->new_x2,
     "y1", ui.selection->new_y1, "y2", ui.selection->new_y2, NULL);
diff -Narpu --exclude '*.orig' --exclude '*.rej' --exclude '*~' xournal-0.4.8.orig/src/xournal.h xournal-0.4.8/src/xournal.h
--- xournal-0.4.8.orig/src/xournal.h	2014-06-28 12:47:20.000000000 -0700
+++ xournal-0.4.8/src/xournal.h	2017-02-09 15:29:08.866517375 -0800
@@ -154,6 +154,7 @@ extern guint predef_bgcolors_rgba[COLOR_
 #define TOOLOPT_ERASER_STANDARD     0
 #define TOOLOPT_ERASER_WHITEOUT     1
 #define TOOLOPT_ERASER_STROKES      2
+#define TOOLOPT_SELECT_PRESERVE     1 // Preserve the aspect ratio of the selection when resizing
 
 extern double predef_thickness[NUM_STROKE_TOOLS][THICKNESS_MAX];