Bug 1793834 - part 8: add preference for number of messages to keep r=handyman

One thing to note is that if the user shrinks this number we won't reclaim any memory - this is because I wanted to avoid doing any more work than necessary when we're logging messages. Let me know if you think this is a problem!

Differential Revision: https://phabricator.services.mozilla.com/D161156

modules/libpref/init/StaticPrefList.yaml

@@ -14671,6 +14671,13 @@
   type: RelaxedAtomicInt32
   value: 0
   mirror: always
+
+# The number of messages of each type to keep for display in
+# about:windows-messages
+- name: widget.windows.messages_to_log
+  type: RelaxedAtomicUint32
+  value: 6
+  mirror: always
 #endif
 
 # Whether to flush the Ole clipboard synchronously.

widget/windows/nsWindowLoggedMessages.cpp

@@ -79,8 +79,6 @@ struct CircularMessageBuffer {
 };
 static std::map<HWND, std::map<UINT, CircularMessageBuffer>> gWindowMessages;
 
-const size_t kNumberOfWindowMessages = 5;
-
 static HWND GetHwndFromWidget(nsIWidget* windowWidget) {
   nsWindow* window = static_cast<nsWindow*>(windowWidget);
   return window->GetWindowHandle();
@@ -264,7 +262,9 @@ void LogWindowMessage(HWND hwnd, UINT event, bool isPreEvent, long eventCounter,
   WindowMessageData messageData = {
       eventCounter, isPreEvent, MakeMessageSpecificData(event, wParam, lParam),
       result, retValue};
-  if (hwndWindowMessages.mMessages.size() < kNumberOfWindowMessages) {
+  uint32_t numberOfMessagesToKeep =
+      Preferences::GetUint("widget.windows.messages_to_log", 6);
+  if (hwndWindowMessages.mMessages.size() < numberOfMessagesToKeep) {
     // haven't reached limit yet
     hwndWindowMessages.mMessages.push_back(std::move(messageData));
   } else {
@@ -272,7 +272,7 @@ void LogWindowMessage(HWND hwnd, UINT event, bool isPreEvent, long eventCounter,
         std::move(messageData);
   }
   hwndWindowMessages.mNextFreeIndex =
-      (hwndWindowMessages.mNextFreeIndex + 1) % kNumberOfWindowMessages;
+      (hwndWindowMessages.mNextFreeIndex + 1) % numberOfMessagesToKeep;
 }
 
 void GetLatestWindowMessages(RefPtr<nsIWidget> windowWidget,
@@ -281,8 +281,9 @@ void GetLatestWindowMessages(RefPtr<nsIWidget> windowWidget,
   const auto& rawMessages = gWindowMessages[hwnd];
   nsTArray<std::pair<WindowMessageDataSortKey, nsCString>>
       sortKeyAndMessageArray;
-  sortKeyAndMessageArray.SetCapacity(rawMessages.size() *
-                                     kNumberOfWindowMessages);
+  sortKeyAndMessageArray.SetCapacity(
+      rawMessages.size() *
+      Preferences::GetUint("widget.windows.messages_to_log", 6));
   for (const auto& eventAndMessage : rawMessages) {
     for (const auto& messageData : eventAndMessage.second.mMessages) {
       nsCString message = MakeFriendlyMessage(