Background blinks when installing/updating add-on

The opaque background blinks when an add-on is installed/updated

Steps to reproduce

1. Install a vanilla JIRA 6.4
2. Navigate to the Find new add-ons page and install an add-on

OR

1. Install a vanilla JIRA 6.4
2. Navigate to the Manage add-ons page and there will be some add-on updates available
3. Update any add-on

Expected Behaviour

1. A progress dialogue pops up, while the rest of the page greys out
2. The progress bar will display install/update progress while the background maintains opaque

Actual Behaviour

1. A progress dialogue pops up, while the rest of the page greys out
2. When install/update is in progress, the background will blink back and forth from opaque to full coloured

Notes

1. Reproduced with Safari, Chrome and Firefox. Didn't test IE