HOBrowserView: Avoid calling super on key events

This workaround is to avoid a couple of undesired behaviors in the `HTMLOutputWindow` when the web page implements keyboard commands such as Git → Browse Annotated File (Blame). Namely, when using the keyboard commands, we would always receive an NSBeep. Also, fixed an issue when these key commands may conflict with menu keys. See comment in the commit for details.

Frameworks/HTMLOutput/src/browser/HOBrowserView.mm

@@ -108,6 +108,45 @@ static void ShowLoadErrorForURL (WebFrame* frame, NSURL* url, NSError* error)
 	}
 }
 
+// ==============
+// = Key Events =
+// ==============
+
+/*
+Since the webView is typically the first responder, the path for key events is as follows:
+
+For keyDown:
+	webView
+	HOBrowserView
+	OakHTMLOutputView
+	NSWindow
+
+For performKeyEquivalent:
+	NSWindow
+	OakHTMLOutputView
+	HOBrowserView
+	webView
+
+A webView default implementation passes all key events, including potential key equivalents (except ESC),
+to the webpage so that it may have a chance to respond. Unfortunately, we cannot know if these events are
+handled so the events are still forwarded down their respective chains as shown above. So to avoid the
+NSBeep when hitting the end of the responder chain, we let HOBrowserView swallow all key events. This is
+safe since performKeyEquivalent: is called first, which leads to another problem: we can pass
+the key event back to the webView (minus the modifier). Therefore, we also terminate the above chain for
+performKeyEquivalent: by overriding the method here and returning just NO. Note: that if none of the views
+in the hierachy returns YES, the key (equivalent) event is then passed to the menus.
+*/
+
+- (BOOL)performKeyEquivalent
+{
+	return NO;
+}
+
+- (void)keyDown:(NSEvent*)anEvent
+{
+
+}
+
 // =========
 // = Swipe =
 // =========