Cannot reproduce.
@Connor McBride can you check if you are using any resource packs and disable them to verify if they cause this issue?
Cannot reproduce on M4 MBP.
@Kathryn Odum @Natalie can you verify your keybind configuration in Options → Controls → Key Binds, for Toggle Fullscreen? It is visible under section 2 - Miscellaneous. Have you bound left mouse click button to Toggle Fullscreen?
This issue is likely possible starting from 26.2-snapshot-4. If you have trouble navigating to the Key Binds screen, either try using keyboard navigation (tab + enter) or launch an older version of the game (e.g. 26.1.2 or 26.2-snapshot-3).
Prior to 26.2-snapshot-4, binding left click mouse button to toggle fullscreen would have been allowed but should not have actually worked. A theory is that you could have had this keybind but not noticed until you launched 26.2 (or 26.2-snapshot-4).
Thank you for your report!
After consideration, the issue is being closed as Working as Intended.
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@Hou_Samuel we can confirm that we see some lag. It appears most problematic the first time after a system restart and less problematic after switching input language multiple times. Our testing has been done on a Macbook with M4 Pro and there the impact is not experienced as severe.
How long lag do you experience? Is it worst the first time you switch?
If you can submit video or images of your experience that could help us understand how bad the situation is.