[gnome-builder] source-view: improve scroll-offset with large value

[Christian Hergert <chergert src gnome org>]

commit 6a03f3e9ccce55410b3a993297d7a1d9c7605275
Author: Christian Hergert <christian hergert me>
Date:   Tue Apr 7 15:13:40 2015 -0700

    source-view: improve scroll-offset with large value
    
    Setting a scroll offset of 999 should keep the current line pinned to the
    center of the screen when possible. We had some strange situations where
    we would actually move a little bit which was jarring.
    
    This doesn't get at the root of the issue, but it does mitigate it enough
    to be much more useful.

 libide/ide-source-view.c |   17 +++++++++++++++++
 1 files changed, 17 insertions(+), 0 deletions(-)
---
diff --git a/libide/ide-source-view.c b/libide/ide-source-view.c
index 2d1ec7f..afacf34 100644
--- a/libide/ide-source-view.c
+++ b/libide/ide-source-view.c
@@ -6410,6 +6410,7 @@ ide_source_view_scroll_to_iter (IdeSourceView     *self,
   GdkRectangle iter_rect;
   gdouble yvalue;
   gdouble xvalue;
+  gdouble orig_yalign;
   gint xoffset;
   gint yoffset;
 
@@ -6474,12 +6475,28 @@ ide_source_view_scroll_to_iter (IdeSourceView     *self,
    * now convert those back to alignments in the real visible area, but leave
    * enough space for an input character.
    */
+  orig_yalign = yalign;
   xalign = xoffset / (gdouble)real_visible_rect.width;
   yalign = yoffset / (gdouble)(real_visible_rect.height + priv->cached_char_height);
 
   yvalue = iter_rect.y - (yalign * real_visible_rect.height);
   xvalue = iter_rect.x - (xalign * real_visible_rect.width);
 
+  /*
+   * FIXME:
+   *
+   * We need to understand better why this phenomina exists.
+   *
+   * 0.0 and 1.0 at visible boundaries creates some interesting artifacts.
+   * This works around the phenomina at to ensure we are pinned inside the
+   * visible area we care about. We probably need to take this into account
+   * in the alignment calculations in a previous step.
+   */
+  if (orig_yalign == 1.0)
+    yvalue += (priv->cached_char_height / 2);
+  else if (orig_yalign == 0.0)
+    yvalue -= (priv->cached_char_height / 2);
+
   frame_clock = gtk_widget_get_frame_clock (GTK_WIDGET (self));
 
   if (animate_scroll)