Black tab previews on webbrowser sometimes

The regression appeared in image #199.

Those are the changes in image #199:

Package changes between 20150424.1 and 20150428
=== Dropped Packages ===
libgo7:armhf

=== Upgraded Packages ===
libmedia-hub-client3:armhf from 3.1.0+15.04.20150420.1-0ubuntu1 to 3.1.0+15.04.20150427.3-0ubuntu1
libmedia-hub-common3:armhf from 3.1.0+15.04.20150420.1-0ubuntu1 to 3.1.0+15.04.20150427.3-0ubuntu1
libusermetricsinput1:armhf from 1.1.1+15.04.20150219-0ubuntu1 to 1.1.1+15.04.20150427-0ubuntu1
libusermetricsoutput1:armhf from 1.1.1+15.04.20150219-0ubuntu1 to 1.1.1+15.04.20150427-0ubuntu1
media-hub from 3.1.0+15.04.20150420.1-0ubuntu1 to 3.1.0+15.04.20150427.3-0ubuntu1
nuntium from 1.4-0ubuntu6 to 1.4+15.04.20150422-0ubuntu1
qtdeclarative5-usermetrics0.1 from 1.1.1+15.04.20150219-0ubuntu1 to 1.1.1+15.04.20150427-0ubuntu1
qtubuntu-media from 0.7.1+15.04.20150420.1-0ubuntu1 to 0.7.1+15.04.20150427.2-0ubuntu1
tzdata from 2015c-1 to 2015d-0ubuntu0.15.04
usermetricsservice:armhf from 1.1.1+15.04.20150219-0ubuntu1 to 1.1.1+15.04.20150427-0ubuntu1

I’m not seeing anything particularly relevant here…

I’m seeing that too on an up-to-date vivid desktop (no overlay PPA) + oxide 1.7. When I downgrade to oxide 1.6, the problem goes away. So it looks like the issue was introduced when updating to oxide 1.7. Earlier builds are not available any longer, but I was able to get packages for 1.7.3 from the phablet-team PPA, and the issue is present already.

The issue seems to be with when the capture is requested. Basically what happens today is something like that:

    Connections {
        target: webview
        onVisibleChanged: {
            if (!webview.visible) {
                webview.grabToImage(function(result) { … })
            }
        }
    }

This used to work with oxide 1.6, but with 1.7 the capture happens too late, it seems that when the webview becomes invisible, the corresponding buffer becomes unavailable, and this results in a completely black image.

I’m now seeing captures again on my arale. After discussion with others, some are seeing a few black tabs out of a number of open tabs, so this seems to be racy (even though I can consistently reproduce on my desktop).