Don't clear buffer pre-context if no new context is being provided
Patch from Jonathan Kew.
Part of fixing:
Mozilla Bug 801410 - avoid inserting dotted-circle for run-initial
Unicode combining characters in "simple" scripts such as Latin
https://bugzilla.mozilla.org/show_bug.cgi?id=801410
diff --git a/src/hb-buffer.cc b/src/hb-buffer.cc
index 1eb513c..06b5c05 100644
--- a/src/hb-buffer.cc
+++ b/src/hb-buffer.cc
@@ -841,7 +841,14 @@
buffer->ensure (buffer->len + item_length * sizeof (T) / 4);
- if (!buffer->len)
+ /* If buffer is empty and pre-context provided, install it.
+ * This check is written this way, to make sure people can
+ * provide pre-context in one add_utf() call, then provide
+ * text in a follow-up call. See:
+ *
+ * https://bugzilla.mozilla.org/show_bug.cgi?id=801410#c13
+ */
+ if (!buffer->len && item_offset > 0)
{
/* Add pre-context */
buffer->clear_context (0);
