When Ubuntu detects that an application is unresponsive, the window loses colour and shows gets a somewhat 'dimmed' appearance. Normally once the application starts responding again the colour comes back and it no longer looks dimmed.
Occasionally the application becomes responsive again but for some reason Ubuntu doesn't seem to notice, and the window stays desaturated. If I remember correctly I think this has happened particularly with Firefox, Thunderbird and LibreOffice but I'm guessing it could happen to any application.
Is there any way to manually restore the saturation without closing and re-opening the application? If I use the menu to open a new window of the same application (e.g. New Window in Firefox, or even if I just open Help -> About), the colour is fine on the new window that pops up, it's only the window that was open when the application stopped responding that is 'stuck' in the dimmed state.