Check if one of your control keys got stuck by pressing them. (might also be one of the other modifiers that does this, not sure right now)
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Plasma 6 Bugs
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
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Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
ctrl isn't stuck because entering and subsequently exiting plasma edit mode makes the default left click behavior work again, at least for some time
I experience this from time to time. It is caused (at least on my case) by meta key being stuck from using shortcuts.
Temporary solution is pressing the meta key alone so it gets unstuck.