100ms blockage when animating the icon grid

Toggling the application icons grid (Super+A) hits a 100ms+ blockage every time it closes.

As shown by mutter!154 (merged) with CLUTTER_SHOW_FPS=1:

*** Performance over 1.0s: 22.17 FPS, update time avg: 10.2ms, peak: 121.7ms
*** Performance over 1.8s: 18.48 FPS, update time avg: 5.7ms, peak: 24.5ms
*** Performance over 1.0s: 41.56 FPS, update time avg: 7.2ms, peak: 113.9ms
*** Performance over 1.0s: 47.86 FPS, update time avg: 6.0ms, peak: 25.8ms
*** Performance over 1.0s: 43.25 FPS, update time avg: 8.3ms, peak: 119.1ms
*** Performance over 1.0s: 43.03 FPS, update time avg: 7.8ms, peak: 120.3ms

and confirmed by mutter!724 (merged) with CLUTTER_DEBUG=backend, which also pinpoints the problem to only when the icon grid closes:

Clutter-Message: 18:20:21.933: [      9847241214]:[BACKEND]:../clutter/clutter/cogl/clutter-stage-cogl.c:276: Missed some frames. Something blocked for over 119ms.
Clutter-Message: 18:20:24.467: [      9849775521]:[BACKEND]:../clutter/clutter/cogl/clutter-stage-cogl.c:276: Missed some frames. Something blocked for over 118ms.
Clutter-Message: 18:20:26.952: [      9852260133]:[BACKEND]:../clutter/clutter/cogl/clutter-stage-cogl.c:276: Missed some frames. Something blocked for over 84ms.
Clutter-Message: 18:20:29.200: [      9854508681]:[BACKEND]:../clutter/clutter/cogl/clutter-stage-cogl.c:276: Missed some frames. Something blocked for over 81ms.
Clutter-Message: 18:20:31.049: [         +810598]:[BACKEND]:../clutter/clutter/cogl/clutter-stage-cogl.c:276: Missed some frames. Something blocked for over 111ms.
Clutter-Message: 18:20:33.033: [         +773122]:[BACKEND]:../clutter/clutter/cogl/clutter-stage-cogl.c:276: Missed some frames. Something blocked for over 110ms.

This does not happen when just toggling the Activities Overview with Super.