47734 - DOM StyleSheets collection inaccurate if retrieved during page load [AltSS]

Status: RESOLVED FIXED

(Core :: DOM: CSS Object Model, defect, P3)

Description

[Tim Hill]

(this requires the latest build with author stylesheet support, or the patch in 
bug 6782)
1. Go to http://www.fas.harvard.edu/~dbaron/css/ssui/
2. While the page is loading, select the View > Use Stylesheet item to show a 
list of alternate stylesheets for the page.  Only some of the stylesheets will 
be shown.
3. Then wait for the entire page to load.  Go to View > Use Stylesheet again to 
view the list.

Actual Results:
- Not all of the alternate stylesheets for the page are shown, even though the 
page is now fully loaded. (If you wait for the page to completely load before 
accessing the menu, all stylesheets ARE shown correctly)

Expected Results:
- All alternate stylesheets should be shown.  

The "Use Stylesheet" menu generates the list of alternate stylesheets using the 
document.styleSheets property each time the menu pops up.  But once you access 
the document.styleSheets collection for the first time (i.e. during page load), 
it seems to keep the same list in memory until the page is reloaded.

Comment 1

[Tim Hill]

Confirming bug because someone else had the same problem... and to test my 
newfound Bugzilla permissions!

Comment 2

[Johnny Stenback (:jst)]

This bug has been marked "future" because the original netscape engineer
working on this is over-burdened. If you feel this is an error, that you or
another known resource will be working on this bug,or if it blocks your work
in some way -- please attach your concern to the bug for reconsideration.

Comment 3

[Hixie (not reading bugmail)]

Taking QA since this is a DOM2 Style bug and I'm supposed to be getting into 
DOM2 Style...

Comment 4

[Hixie (not reading bugmail)]

(marking alternate style sheet bugs for easy tracking...)

Comment 5

[Hixie (not reading bugmail)]

Taking QA Contact on all open or unverified DOM Style bugs...

Comment 6

[Ronald van Kuijk]

WORKSFORME on Linux build 2001052621

Please verify...

Comment 7

[Johnny Stenback (:jst)]

I still see this problem.

Comment 8

[Fabian Guisset]

*** Bug 111765 has been marked as a duplicate of this bug. ***

Comment 9

[Fabian Guisset]

http://bugzilla.mozilla.org/showattachment.cgi?attach_id=59069
is a testcase for this bug as well (from bug 111765)

Comment 10

[Boris Zbarsky [:bzbarsky]]

*** Bug 107118 has been marked as a duplicate of this bug. ***

Comment 11

[Boris Zbarsky [:bzbarsky]]

My comments from bug 111765:

The problem is the following:

1)  We defer alternate sheet loads.  So when we create the nsDOMStyleSheetList
    and it calls GetNumberOfStyleSheets the first time it gets 0.
2)  The stylesheet list caches the 0 and only updates it if the StyleSheetAdded
    callback is called.

This callback is never called if all the pending sheets are alternate sheets. 
Worse, yet, if the remaining sheets are not all alternates there will be only
_one_ call to StyleSheetAdded, since the CSS loader will only set notification
to true on the last pending sheet.  See

http://lxr.mozilla.org/seamonkey/source/content/html/style/src/nsCSSLoader.cpp#798

There is no real explanation for why we do it that way and I'm not sure what the
performance (or correctness) impact would be of notifying on all sheets here. 
ccing pierre.

The two solutions I see would be the following:

1)  Have the CSS loader_always_ file a notification on pending sheets (even if
    they are all alternates) and have the nsDOMStyleSheetList set mLength to -1
    instead of incrementing it when StyleSheetAdded is called
2)  Have the CSS loader notify on all stylesheet insertions

Pierre?  Thoughts?

Comment 12

[Pierre Saslawsky]

The problem is that we have two kinds of observers but only one kind of 
notification. The HTMLContentSink needs to be notified when a doc sheet is 
inserted in the document in order to rebuild everything.  The nsDOMStyleSheetList 
needs to be notified when a sheet is linked to the document (which does not mean 
that it will later be inserted in the cascade) in order to update its count.

As Boris pointed out, the current notifications only cover the first case.
A solution could be for the document to be notified when a sheet is loaded (see 
the StyleSheetLoaded notification in nsICSSLoaderObserver) and forward this 
notification to its |nsIDocumentObserver|s.  It would allow nsDOMStyleSheetList 
to set mLength to -1.  The other observers could just ignore the notification.

We don't want to send the StyleSheetAdded notification on all stylesheets.  It is 
incorrect for alternate sheets to do that and it would hurt the performance quite 
a bit.

Comment 13

[Boris Zbarsky [:bzbarsky]]

Fixed for 1.3a by checkin for bug 107567