[evince/gnome-3-36] libdocument: Stop trusting phsyical dimensions from monitors

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

commit 10edc783fb8e8b9904fc6ef8924e64b0af843c9e
Author: Mike Vastola <mike vasto la>
Date:   Thu Sep 10 16:13:04 2020 -0400

    libdocument: Stop trusting phsyical dimensions from monitors
    
    Per gtk#3115#note_904622, no gdk_*_get_*_mm methods may be trusted, so
    use defaults.
    
    Fixes #1403

 libdocument/ev-document-misc.c | 36 +++++++++++++++++++-----------------
 1 file changed, 19 insertions(+), 17 deletions(-)
---
diff --git a/libdocument/ev-document-misc.c b/libdocument/ev-document-misc.c
index 1943ff1c..1d0acc5c 100644
--- a/libdocument/ev-document-misc.c
+++ b/libdocument/ev-document-misc.c
@@ -540,9 +540,8 @@ G_GNUC_END_IGNORE_DEPRECATIONS
  * ev_document_misc_get_widget_dpi:
  * @widget: a #GtkWidget
  *
- * Returns DPI for monitor on which given widget has been realized.
- * Returns DPI of primary monitor or DPI of first monitor in the list inside
- * of GdkDisplay if the widget has not been realized yet.
+ * Returns sensible guess for DPI of monitor on which given widget has been
+ * realized. If HiDPI display, use 192, else 96.
  * Returns 96 as fallback value.
  *
  * Returns: DPI as gdouble
@@ -554,7 +553,7 @@ ev_document_misc_get_widget_dpi (GtkWidget *widget)
        GdkDisplay   *display;
        GdkMonitor   *monitor;
        GdkWindow    *window;
-       gdouble       dp, di;
+       gboolean      is_landscape;
 
        display = gtk_widget_get_display (widget);
        window = gtk_widget_get_window (widget);
@@ -566,22 +565,25 @@ ev_document_misc_get_widget_dpi (GtkWidget *widget)
                        monitor = gdk_display_get_monitor (display, 0);
        }
 
-       if (monitor != NULL) {
-               /* diagonal in pixels */
-               gdk_monitor_get_geometry (monitor, &geometry);
-               dp = hypot (geometry.width, geometry.height);
-               if (dp == 0)
-                       return 96;
+       /* The only safe assumption you can make, on Unix-like/X11 and
+        * Linux/Wayland, is to always set the DPI to 96, regardless of
+        * physical/logical resolution, because that's the only safe
+        * guarantee we can make.
+        * https://gitlab.gnome.org/GNOME/gtk/-/issues/3115#note_904622 */
+       if (monitor == NULL)
+               return 96;
 
-               /* diagonal in inches */
-               di = hypot (gdk_monitor_get_width_mm (monitor), gdk_monitor_get_height_mm (monitor)) / 25.4;
-               if (di == 0)
-                       return 96;
+       gdk_monitor_get_geometry (monitor, &geometry);
+       is_landscape = geometry.width > geometry.height;
 
-               return (dp / di);
-       } else {
+       /* DPI is 192 if height ≥ 1080 and the orientation is not portrait,
+        * which is, incidentally, how GTK detects HiDPI displays and set a
+        * scaling factor for the logical output
+        * https://gitlab.gnome.org/GNOME/gtk/-/issues/3115#note_904622 */
+       if (is_landscape && geometry.height >= 1080)
+               return 192;
+       else
                return 96;
-       }
 }
 
 /* Returns a locale specific date and time representation */