[lock screen] Don't blink the text field's insertion point when invisible
Having the insertion point blink seems to use quite a lot of CPU resources, which is a separate issue. But having it blink when the thing it's displayed inside is hidden does not make sense. Let's have it not do that.
This workaround was found by Fabian Vogt.
BUG: 347772 FIXED-IN: 5.21