Bug 1531226 - Add support for alwaysontop windows for macOS. r=mstange

Differential Revision: https://phabricator.services.mozilla.com/D50275

--HG--
extra : moz-landing-system : lando

widget/cocoa/nsCocoaWindow.h

@@ -389,6 +389,7 @@ class nsCocoaWindow final : public nsBaseWidget, public nsPIWidgetCocoa {
   bool mInReportMoveEvent;  // true if in a call to ReportMoveEvent().
   bool mInResize;           // true if in a call to DoResize().
   bool mWindowTransformIsIdentity;
+  bool mAlwaysOnTop;
 
   int32_t mNumModalDescendents;
   InputContext mInputContext;

widget/cocoa/nsCocoaWindow.mm

@@ -154,6 +154,7 @@ nsCocoaWindow::nsCocoaWindow()
       mInReportMoveEvent(false),
       mInResize(false),
       mWindowTransformIsIdentity(true),
+      mAlwaysOnTop(false),
       mNumModalDescendents(0),
       mWindowAnimationBehavior(NSWindowAnimationBehaviorDefault) {
   if ([NSWindow respondsToSelector:@selector(setAllowsAutomaticWindowTabbing:)]) {
@@ -302,6 +303,7 @@ nsresult nsCocoaWindow::Create(nsIWidget* aParent, nsNativeWidget aNativeParent,
 
   mParent = aParent;
   mAncestorLink = aParent;
+  mAlwaysOnTop = aInitData->mAlwaysOnTop;
 
   // Applications that use native popups don't want us to create popup windows.
   if ((mWindowType == eWindowType_popup) && UseNativePopupWindows()) return NS_OK;
@@ -482,6 +484,10 @@ nsresult nsCocoaWindow::CreateNativeWindow(const NSRect& aRect, nsBorderStyle aB
     [mWindow setOpaque:YES];
   }
 
+  if (mAlwaysOnTop) {
+    [mWindow setLevel:NSFloatingWindowLevel];
+  }
+
   [mWindow setContentMinSize:NSMakeSize(60, 60)];
   [mWindow disableCursorRects];