Add per_monitor_dpi experimental setting

Calculating the screen dpi on a per-monitor basis can cause
inconsistencies if multiple monitors with slightly different dpis are
used and in some cases it might not be the expected behaviour.

As such, the per-monitor dpi calculation was changed from a default
fallback to an opt in experimental feature and the default value of 96
will be used for the dpi if Xft.dpi is not set.

In the future, depending on how we decide to continue, we can either
move this setting in the global configuration section and fall back to
the X11 display dpi as the default or simply always use the per-monitor
dpi calculation. But to preserve backwards compatibility, this decision
can wait until the next major release.

CHANGELOG.md

@@ -10,6 +10,8 @@
 - Support for per-urgency frame colours
 - `markup` setting for more fine-grained control over how markup is handled
 - `history_ignore` rule action to exclude a notification from being added to the history
+- Support for setting the dpi value dunst will use for font rendering via the `Xft.dpi` X resource
+- Experimental support for per-monitor dpi calculation
 
 ### Changed
 - Text and icons are now centred vertically

dunstrc

@@ -181,6 +181,13 @@
     # Always run rule-defined scripts, even if the notification is suppressed
     always_run_script = true
 
+# Experimental features that may or may not work correctly. Do not expect them
+# to have a consistent behaviour across releases.
+[experimental]
+    # Calculate the dpi to use on a per-monitor basis.
+    # Please note that this setting will not work if Xft.dpi X resource is set.
+    per_monitor_dpi = false
+
 [frame]
     width = 3
     color = "#aaaaaa"

src/settings.c

@@ -97,6 +97,12 @@ void load_settings(char *cmdline_config_path)
         load_ini_file(config_file);
 #endif
 
+        settings.per_monitor_dpi = option_get_bool(
+                "experimental",
+                "per_monitor_dpi", NULL, false,
+                ""
+        );
+
         settings.font = option_get_string(
                 "global",
                 "font", "-fn", font,

src/settings.h

@@ -14,6 +14,7 @@ enum markup_mode { MARKUP_NULL, MARKUP_NO, MARKUP_STRIP, MARKUP_FULL };
 
 typedef struct _settings {
         bool print_notifications;
+        bool per_monitor_dpi;
         enum markup_mode markup;
         bool stack_duplicates;
         bool hide_duplicate_count;

src/x11/screen.c

@@ -56,7 +56,6 @@ static double get_xft_dpi_value()
                 }
                 XrmDestroyDatabase(xDB);
         }
-
         return dpi;
 }
 
@@ -106,7 +105,7 @@ void x_update_screens()
         XRRFreeMonitors(m);
 }
 
-static double autodetect_dpi(screen_info *scr)
+static int autodetect_dpi(screen_info *scr)
 {
         return (double)scr->dim.h * 25.4 / (double)scr->dim.mmh;
 }
@@ -256,9 +255,12 @@ sc_cleanup:
 double get_dpi_for_screen(screen_info *scr)
 {
         double dpi = 0;
-        if ((dpi = get_xft_dpi_value()) || (dpi = autodetect_dpi(scr)))
+        if ((dpi = get_xft_dpi_value()))
                 return dpi;
-        return 0;
+        else if (settings.per_monitor_dpi && (dpi = autodetect_dpi(scr)))
+                return dpi;
+        else
+                return 96;
 }
 
 /*