GTK: Fix fetching the frame border width on GTK 3.20

GTK 3.20 introduced a sub-node named "border" to the frame, requiring
to lookup the border on that sub-node rather than on the frame itself.

Unfortunately supporting this requires to be built against GTK 3.20, as
it requires API this version introduced.  This means that a build
against an older version won't be able to get the popup sizing right if
running on 3.20.  To mitigate this, add reasonable fallback defaults
when running 3.20 but built on an earlier version, to try and avoid

X-Scintilla-Bug-URL: https://sourceforge.net/p/scintilla/bugs/1825/
X-Scintilla-Commit-ID: 83d56b1fc63a206e4c1b776f5991b3b17ccb3473
2px-scrolling on GTK 3.20's default theme.

scintilla/gtk/PlatGTK.cxx

@@ -1534,12 +1534,34 @@ PRectangle ListBoxX::GetDesiredRect() {
 		int row_height = GetRowHeight();
 #if GTK_CHECK_VERSION(3,0,0)
 		GtkStyleContext *styleContextFrame = gtk_widget_get_style_context(PWidget(frame));
-		GtkBorder padding, border;
+		GtkBorder padding, border, border_border = { 0, 0, 0, 0 };
 		gtk_style_context_get_padding(styleContextFrame, GTK_STATE_FLAG_NORMAL, &padding);
 		gtk_style_context_get_border(styleContextFrame, GTK_STATE_FLAG_NORMAL, &border);
+
+#	if GTK_CHECK_VERSION(3,20,0)
+		// on GTK 3.20 the frame border is in a sub-node "border".
+		// Unfortunately we need to be built against 3.20 to be able to support this, as it requires
+		// new API.
+		GtkStyleContext *styleContextFrameBorder = gtk_style_context_new();
+		GtkWidgetPath *widget_path = gtk_widget_path_copy(gtk_style_context_get_path(styleContextFrame));
+		gtk_widget_path_append_type(widget_path, GTK_TYPE_BORDER); // dummy type
+		gtk_widget_path_iter_set_object_name(widget_path, -1, "border");
+		gtk_style_context_set_path(styleContextFrameBorder, widget_path);
+		gtk_widget_path_free(widget_path);
+		gtk_style_context_get_border(styleContextFrameBorder, GTK_STATE_FLAG_NORMAL, &border_border);
+		g_object_unref(styleContextFrameBorder);
+#	else // < 3.20
+		if (gtk_check_version(3, 20, 0) == NULL) {
+			// default to 1px all around as it's likely what it is, and so we don't miss 2px height
+			// on GTK 3.20 when built against an earlier version.
+			border_border.top = border_border.bottom = border_border.left = border_border.right = 1;
+		}
+#	endif
+
 		height = (rows * row_height
 		          + padding.top + padding.bottom
 		          + border.top + border.bottom
+		          + border_border.top + border_border.bottom
 		          + 2 * gtk_container_get_border_width(GTK_CONTAINER(PWidget(list))));
 #else
 		height = (rows * row_height
@@ -1560,6 +1582,7 @@ PRectangle ListBoxX::GetDesiredRect() {
 #if GTK_CHECK_VERSION(3,0,0)
 		rc.right += (padding.left + padding.right
 		             + border.left + border.right
+		             + border_border.left + border_border.right
 		             + 2 * gtk_container_get_border_width(GTK_CONTAINER(PWidget(list))));
 #else
 		rc.right += 2 * (PWidget(frame)->style->xthickness