If you are in gamemode creative and you resize the window, a stone block gets placed in the "Destroy Item" slot. You can get rid of it if you reopen your inventory or switch to another tab.
I've also seen other items like armor in that slot after rescaling.
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However, this issue has been closed as a Duplicate of MC-132063.
It has been linked to this report. If you have additional information, please add it to the duplicated report.
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