[balsa] html: Fix HTML rendering with webkit2gtk-4.0 < 2.20

[Peter Bloomfield <peterb src gnome org>]

commit 3423ca9bf48e675007b7fb1bd61b50aa97d14890
Author: Albrecht Dreß <albrecht dress arcor de>
Date:   Sat Jul 7 16:36:49 2018 -0400

    html: Fix HTML rendering with webkit2gtk-4.0 < 2.20
    
    * README: document use of G_MESSAGES_DEBUG=…
    * libbalsa/html.c (lbh_navigation_policy_decision):
      allow the rendering of “about:blank”.
    
    Signed-off-by: Peter Bloomfield <PeterBloomfield bellsouth net>

 ChangeLog       |  8 ++++++++
 README          | 14 ++++++++++++++
 libbalsa/html.c | 12 ++++++++----
 3 files changed, 30 insertions(+), 4 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 8e0474a52..c746fed06 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,11 @@
+2018-07-07  Albrecht Dreß <albrecht dress arcor de>
+
+       Fix HTML rendering with webkit2gtk-4.0 < 2.20
+
+       * README: document use of G_MESSAGES_DEBUG=…
+       * libbalsa/html.c (lbh_navigation_policy_decision):
+         allow the rendering of “about:blank”.
+
 2018-07-07  Peter Bloomfield  <pbloomfield bellsouth net>
 
        Use g_debug() instead of conditionally compiled printing
diff --git a/README b/README
index 0db1bc8ea..701edd88e 100644
--- a/README
+++ b/README
@@ -235,6 +235,20 @@ directory. It should contain the following:
 filename=/path/to/your/browser
 
 
+Debugging:
+---------
+Set the environment variable G_MESSAGES_DEBUG to print debugging
+information to the console.  The value shall be either a space-
+separated list of log domains, or the special value "all".  The
+following custom domains are implemented in Balsa:
+- libnetclient: low-level network IO.  Warning: the output may contain
+        plain-text passwords.
+- imap: IMAP server interaction.  Warning: the output may contain
+        plain-text passwords.
+- crypto: GnuPG and S/MIME crypto operations
+- html: HTML rendering with webkit2
+
+
 Reporting Bugs:
 --------------
        See http://balsa.gnome.org/bugs.html for instructions.
diff --git a/libbalsa/html.c b/libbalsa/html.c
index 1e5e2f500..307194e0a 100644
--- a/libbalsa/html.c
+++ b/libbalsa/html.c
@@ -44,13 +44,13 @@
 #include <string.h>
 #include <glib/gi18n.h>
 
+#ifdef HAVE_HTML_WIDGET
+
 #ifdef G_LOG_DOMAIN
 #  undef G_LOG_DOMAIN
 #endif
 #define G_LOG_DOMAIN "html"
 
-#ifdef HAVE_HTML_WIDGET
-
 /*
  * Used by all HTML widgets
  *
@@ -281,8 +281,12 @@ lbh_navigation_policy_decision(WebKitPolicyDecision * decision,
         g_debug("%s clicked %s", __func__, uri);
         (*info->clicked_cb) (uri);
     default:
-        g_debug("%s uri %s, type %d, ignored", __func__, uri, navigation_type);
-        webkit_policy_decision_ignore(decision);
+        if (g_ascii_strcasecmp(uri, "about:blank") != 0) {
+            g_debug("%s uri %s, type %d, ignored", __func__, uri, navigation_type);
+               webkit_policy_decision_ignore(decision);
+        } else {
+               g_debug("%s uri %s, type %d loaded", __func__, uri, navigation_type);
+        }
     }
 }