[gtk] Range: Add should_invert_move() for scrolls & keys

[Daniel Boles <dboles src gnome org>]

commit bc2a38a59e7e504612dda668ea2d9ed16dc03c5e
Author: Daniel Boles <dboles src gnome org>
Date:   Tue Apr 17 21:44:16 2018 +0100

    Range: Add should_invert_move() for scrolls & keys
    
    This will be used in subsequent commits to fix the sign by which the
    value is changed in response to directional scroll or keypress events.
    
    The idea is: you have a movement to make – in the form of a delta that
    follows widget directions, i.e. −1 means left or up, +1 means right or
    down – and you want to know whether that delta needs to be inverted in
    order to produce the intuitively expected directional change of :value.
    
    The existing should_invert() is not sufficient: it just determines
    whether to invert visually, but we need more nuance than that for input.
    
    To answer that – while not doubling up the work for scrolls and keys – I
    add a helper should_invert_move(), which considers other relevant state:
    
     • A parallel movement on priv->orientation should just use the existing
       should_invert(), which already worked OK for this case (not others).
    
     • Movements on the other orientation now depend on priv->orientation:
    
        ◦ For a horizontal Range, always invert, so up (i.e. −ve in terms of
          widget coords) always means increase value & vice-versa. This was
          done in get_wheel_delta(), but move it here for use with keys too.
    
        ◦ For a vertical Range, ignore :invert as it’s only relevant to the
          parallel orientation. Do not care about text direction here either
          as RTL locales do not invert number lines, Cartesian plots, etc.
    
    This returns TRUE if the delta should be inverted before applying to the
    value, and we can now use this function in both scroll and key handlers.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=407242
    https://bugzilla.gnome.org/show_bug.cgi?id=791802

 gtk/gtkrange.c | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)
---
diff --git a/gtk/gtkrange.c b/gtk/gtkrange.c
index 67b94f7da2..29082d06e9 100644
--- a/gtk/gtkrange.c
+++ b/gtk/gtkrange.c
@@ -693,6 +693,24 @@ should_invert (GtkRange *range)
     return priv->inverted;
 }
 
+static gboolean
+should_invert_move (GtkRange       *range,
+                    GtkOrientation  move_orientation)
+{
+  GtkRangePrivate *priv = gtk_range_get_instance_private (range);
+
+  /* If the move is parallel to the range, use general check for inversion */
+  if (move_orientation == priv->orientation)
+    return should_invert (range);
+
+  /* H range/V move: Always invert, so down/up always dec/increase the value */
+  if (priv->orientation == GTK_ORIENTATION_HORIZONTAL)
+    return TRUE;
+
+  /* V range/H move: Left/right always dec/increase the value */
+  return FALSE;
+}
+
 static void
 update_highlight_position (GtkRange *range)
 {