From 3ffa329f20612e9d69cbbe963e599d2f6bd8eba0 Mon Sep 17 00:00:00 2001
From: Marc Durdin <marc@keyman.com>
Date: Wed, 11 Apr 2018 11:52:47 +0700
Subject: [PATCH] Pressing backspace corrupts cached context with SMP
 characters

---
 windows/src/engine/keyman32/appint/appint.cpp | 16 ++++++++++++++--
 windows/src/engine/keyman32/appint/appint.h   |  1 +
 2 files changed, 15 insertions(+), 2 deletions(-)

diff --git a/windows/src/engine/keyman32/appint/appint.cpp b/windows/src/engine/keyman32/appint/appint.cpp
index aaccab9c3c..cd067e8f9e 100644
--- a/windows/src/engine/keyman32/appint/appint.cpp
+++ b/windows/src/engine/keyman32/appint/appint.cpp
@@ -84,8 +84,11 @@ WCHAR *AppContext::BufMax(int n)  // Used only by IMX DLLs
 
 void AppContext::Delete()
 {
-	if(CharIsDeadkey())	pos -= 2;
-
+  if (CharIsDeadkey()) {
+    pos -= 2;
+  } else if (CharIsSurrogatePair()) {
+    pos--;
+  }
 	//SendDebugMessageFormat(0, sdmAIDefault, 0, "AppContext::Delete");
 
 	if(pos > 0) pos--;
@@ -144,6 +147,15 @@ BOOL AppContext::CharIsDeadkey()
 		   CurContext[pos-2] == CODE_DEADKEY;
 }
 
+BOOL AppContext::CharIsSurrogatePair()
+{
+  if (pos < 2) // low_surrogate, high_surrogate
+    return FALSE;
+
+  return Uni_IsSurrogate1(CurContext[pos - 2]) &&
+    Uni_IsSurrogate2(CurContext[pos - 1]);
+}
+
 /* AppActionQueue */
 
 AppActionQueue::AppActionQueue()
diff --git a/windows/src/engine/keyman32/appint/appint.h b/windows/src/engine/keyman32/appint/appint.h
index 3378327a50..0b0d9025fe 100644
--- a/windows/src/engine/keyman32/appint/appint.h
+++ b/windows/src/engine/keyman32/appint/appint.h
@@ -95,6 +95,7 @@ public:
 	WCHAR *BufMax(int n);
 	WCHAR *Buf(int n);
 	BOOL CharIsDeadkey();
+  BOOL CharIsSurrogatePair();
 };
 
 class AppContextWithStores : public AppContext   // I4978