[evince] libview: Adjust preview size and contents' position

[Germán Poo-Caamaño <gpoo src gnome org>]

commit f95c24b96de0024f5888e1b11f787ad1432e2263
Author: Henry Gebhardt <hsggebhardt gmail com>
Date:   Sun Dec 2 22:25:18 2018 -0500

    libview: Adjust preview size and contents' position
    
    Makes the preview as wide as possible, and tries to put the link
    slightly above the center of the preview.
    
    Partially fixes #662

 libview/ev-view.c | 54 ++++++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 48 insertions(+), 6 deletions(-)
---
diff --git a/libview/ev-view.c b/libview/ev-view.c
index a40b853e..bb483119 100644
--- a/libview/ev-view.c
+++ b/libview/ev-view.c
@@ -118,6 +118,10 @@ typedef struct {
 #define ANNOT_POPUP_WINDOW_DEFAULT_HEIGHT 150
 #define ANNOTATION_ICON_SIZE 24
 
+#define LINK_PREVIEW_PAGE_RATIO 1.0 / 3.0     /* Size of popover with respect to page size */
+#define LINK_PREVIEW_HORIZONTAL_LINK_POS 0.5  /* as fraction of preview width */
+#define LINK_PREVIEW_VERTICAL_LINK_POS 0.3    /* as fraction of preview height */
+
 /*** Scrolling ***/
 static void       view_update_range_and_current_page         (EvView             *view);
 
@@ -5062,15 +5066,53 @@ link_preview_show_thumbnail (GdkPixbuf *pixbuf,
                             EvView *view)
 {
        GtkWidget *popover = view->link_preview.popover;
-       gdouble         left = view->link_preview.left;
-       gdouble         top = view->link_preview.top;
-       int             width = MIN(gdk_pixbuf_get_width (pixbuf) - left, gtk_widget_get_allocated_width 
(GTK_WIDGET (view)));
-       int             height = MIN (gdk_pixbuf_get_height (pixbuf) - top, 365);
-       GdkPixbuf       *thumbnail_slice;
+       GdkPixbuf *thumbnail_slice;
        GtkWidget *image_view;
+       gdouble    x, y;   /* position of the link on destination page */
+       gint       pwidth, pheight;  /* dimensions of destination page */
+       gint       vwidth, vheight;  /* dimensions of main view */
+       gint       width, height;    /* dimensions of popup */
+       gint       left, top;
+
+       x = view->link_preview.left;
+       y = view->link_preview.top;
+
+       pwidth = gdk_pixbuf_get_width (pixbuf);
+       pheight = gdk_pixbuf_get_height (pixbuf);
+
+       vwidth = gtk_widget_get_allocated_width (GTK_WIDGET (view));
+       vheight = gtk_widget_get_allocated_height (GTK_WIDGET (view));
+
+       /* Horizontally, we try to display the full width of the destination
+        * page. This is needed to make the popup useful for two-column papers.
+        * Vertically, we limit the height to maximally LINK_PREVIEW_PAGE_RATIO
+        * of the main view. The idea is avoid the popup dominte the main view,
+        * and the reader can see context both in the popup and the main page.
+        */
+       width = MIN (pwidth, vwidth);
+       height = MIN (pheight, (int)(vheight * LINK_PREVIEW_PAGE_RATIO));
+
+       /* Position on the destination page that will be in the top left
+        * corner of the popup. We choose the link destination to be centered
+        * horizontally, and slightly above the center vertically. This is a
+        * compromise given that a link contains only (x,y) information for a
+        * single point, and some links have their (x,y) point to the top left
+        * of their main content (e.g. section headers, bibliographic
+        * references, footnotes, and tables), while other links have their
+        * (x,y) point to the center right of the main contents (e.g.
+        * equations). Also, figures usually have their (x,y) point to the
+        * caption below the figure, so seeing a little of the figure above is
+        * often enough to remind the reader of the rest of the figure.
+        */
+       left = x - width * LINK_PREVIEW_HORIZONTAL_LINK_POS;
+       top = y - height * LINK_PREVIEW_VERTICAL_LINK_POS;
+
+       /* link preview destination should stay within the destination page: */
+       left = MIN (MAX (0, left), pwidth - width);
+       top = MIN (MAX (0, top), pheight - height);
 
        thumbnail_slice = gdk_pixbuf_new_subpixbuf (pixbuf,
-                                                   (int)left, (int)top,
+                                                   left, top,
                                                    width, height);
        image_view = gtk_image_new_from_pixbuf (thumbnail_slice);