Reproduction:

Use the dialogs test (tests/manual/dialogs). Click on 'Show modal'. The dialog is not being refreshed - try typing in it. Only when one clicks on the dialog, the typed characters become visible.

This is due to several expose events being put in the system event queue for the window, where the window is still not visible - this results in the exposed property of the window being false, which results in a drop in window sync request.