aura-x11: Add custom web cursor support.

ui/aura/root_window.h

Index: ui/aura/root_window.h
diff --git a/ui/aura/root_window.h b/ui/aura/root_window.h
index c84d6977dfc48b3897799e1cf222749690cae16e..53854faaf11963b2f731fba780cac856cb084d80 100644
--- a/ui/aura/root_window.h
+++ b/ui/aura/root_window.h
@@ -83,9 +83,16 @@ class AURA_EXPORT RootWindow : public ui::CompositorDelegate,
   // Sets the currently-displayed cursor. If the cursor was previously hidden
   // via ShowCursor(false), it will remain hidden until ShowCursor(true) is
   // called, at which point the cursor that was last set via SetCursor() will be
-  // used.
+  // used. If the cursor type is kCursorCustom, then the caller has to call
+  // SetCustomCursor instead to have the custom cursor displayed.
   void SetCursor(gfx::NativeCursor cursor);
 
+  // Sets a custom cursor for the display. If the cursor was previously hidden
+  // via ShowCursor(false), it will remain hidden. However, once
+  // ShowCursor(true) is called, this custom cursor will not be used until an
+  // explicit call to SetCustomCursor is made.

[Daniel Erat, 2012/02/24 17:17:12]

these semantics seem odd to me.  what about making
RootWindowHostLinux::SetCustomCursor() instead save the custom cursor to a
member variable and set |current_cursor_| to kCursorCustom, and making
RootWindowHostLinux::SetCursorInternal() maybe additionally take a
gfx::PlatformCursor for the kCursorCustom case?  that way, ShowCursor() and
HideCursor() will work similarly for both native and custom cursors.  is the
concern that the X cursor will be destroyed if we don't immediately assign it to
a window?  (i think the server does reference counting for cursors.)

[sadrul, 2012/02/24 17:38:19]

My concern is that by the time ShowCursor gets called, the PlatformCursor may
have been destroyed (e.g. the page navigated or was closed etc.), and is not
valid anymore. This can be fixed if we reach into RootWindow from
WebCursor::CleanupPlatformData and update the member variable for the
platform-cursor appropriately. Does that sounds better than this?

[Daniel Erat, 2012/02/24 18:05:47]

At least the X case, I think that the underlying cursor may remain usable as
long as it's assigned to a window, even after XFreeCursor() is called (at least,
I think that cursor IDs returned by XCreateFontCursor() work this way).  There's
still the downside that we'll lose the reference if the PlatformCursor is
cleaned up after a call to HideCursor(), I guess.

Would adding our own wrapper around the underlying X cursor and making it be
reference-counted be too gross?  (Possibly.)

[sadrul, 2012/02/24 18:30:53]

I considered doing that at one point, but that seemed like too much work for not
much gain :) Having said that, having a cache for custom X cursors probably
won't be all that much work (there is a cache for standard cursors in x11_util,
we can probably reuse that to some extent).

[sadrul, 2012/02/24 19:46:30]

Went ahead and did the caching (not too much work!). :-)

+  void SetCustomCursor(const gfx::PlatformCursor& cursor);
+
   // Shows or hides the cursor.
   void ShowCursor(bool show);