When popup/menu window is present - Xfwm4 can't make other actions.
In thunderbird, user did right click on Inbox, popup menu is open
thunderbird is freeze for internal operation (for example compact data, or run filter) and until thunderbird not finish own internal operation whole UI (all - windows, panels, trays) is not react on any user actions.
And this is not performance issue.
The desktop has nothing to do with this, this is purely the toolkit/application responsible for this.
(In reply to Olivier Fourdan from comment #2)
> The desktop has nothing to do with this, this is purely the
> toolkit/application responsible for this.
No. All user actions is unworkable - hot keys, mouse clicks all applications have no reactions, but user can see as other graphics applications is work, for example in window xfce4-taskmanager graphs is redrawing and list of processes is updating without lags, and cpu is not under havy load.
This bug can be seen on other applcations (if they have similar internal whole locks, such as thunderbird) when have present popup window.
This behaivor may be linked with internals of X Window system.
But I think possible to create workaround in WM.
For example run Libreoffice, click on menu "File" (popup window) and press ALT+F4 (Close window) - you'll see window is steel opened.
That's X11 grabs, not something fixable from the WM/desktop, you'll have to switch to Wayland if you want to get rid of grabs (somehow) and use only Wayland native applications with a Wayland compositor (which xfce is not).