It looks like the EGL window is not connected to XWindow. It may happens when a window is mapped/unmapped, GdkWindow stays the same but underlying XWindow is changed. In such case we don't regenerate the EGL window. I'll try to finish Bug 1730533 asap as it solves such cases.
Bug 1737078 Comment 5 Edit History
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It looks like the EGL window is not connected to XWindow. It may happen when a window is mapped/unmapped, GdkWindow stays the same but underlying XWindow is changed. In such case we don't regenerate the EGL window. I'll try to finish Bug 1730533 asap as it solves such cases.