Bug 1104875. Ellipsize long unresponsive script locations so they don't make the slow script dialog too big. r=peterv

dom/base/nsGlobalWindow.cpp

@@ -70,6 +70,7 @@
 #include "AudioChannelService.h"
 #include "MessageEvent.h"
 #include "nsAboutProtocolUtils.h"
+#include "nsCharTraits.h" // NS_IS_HIGH/LOW_SURROGATE
 
 // Interfaces Needed
 #include "nsIFrame.h"
@@ -11073,7 +11074,32 @@ nsGlobalWindow::ShowSlowScriptDialog()
   // Append file and line number information, if available
   if (filename.get()) {
     nsXPIDLString scriptLocation;
+    // We want to drop the middle part of too-long locations.  We'll
+    // define "too-long" as longer than 60 UTF-16 code units.  Just
+    // have to be a bit careful about unpaired surrogates.
     NS_ConvertUTF8toUTF16 filenameUTF16(filename.get());
+    if (filenameUTF16.Length() > 60) {
+      // XXXbz Do we need to insert any bidi overrides here?
+      size_t cutStart = 30;
+      size_t cutLength = filenameUTF16.Length() - 60;
+      MOZ_ASSERT(cutLength > 0);
+      if (NS_IS_LOW_SURROGATE(filenameUTF16[cutStart])) {
+        // Don't truncate before the low surrogate, in case it's preceded by a
+        // high surrogate and forms a single Unicode character.  Instead, just
+        // include the low surrogate.
+        ++cutStart;
+        --cutLength;
+      }
+      if (NS_IS_LOW_SURROGATE(filenameUTF16[cutStart + cutLength])) {
+        // Likewise, don't drop a trailing low surrogate here.  We want to
+        // increase cutLength, since it might be 0 already so we can't very well
+        // decrease it.
+        ++cutLength;
+      }
+
+      // Insert U+2026 HORIZONTAL ELLIPSIS
+      filenameUTF16.Replace(cutStart, cutLength, NS_LITERAL_STRING("\x2026"));
+    }
     const char16_t *formatParams[] = { filenameUTF16.get() };
     rv = nsContentUtils::FormatLocalizedString(nsContentUtils::eDOM_PROPERTIES,
                                                "KillScriptLocation",