Bukkit

Testing GeyserMC recently, most of the commands work perfectly, except icon menus. Pocket edition players can click on the item and then click on the empty slot in their inventory, the item is simply transferred to their inventory and becomes a normal item. If there's any way to improve compatibility with GeyserMC, it would be very welcome, as for now, I have to find a workaround how to disable all icon menus for players logging in using pocket edition and instead make a custom set of commands just for them. Also BOOK and RAW_BOOK commands just don't work at all for bedrock clients, not a big deal. 

I don't know if i can do something, also because as you said, it's GeyserMC that act as intermediary. The best i can do, is to cancel the event with spigot api, more that this i don't know. Does it happen with other iconmenu plugin? Does it happen with only MyCommand and GeyserMC installed? Maybe there is an incompatibility between multiple plugin using IconMenus.

Very likely an issue with most gui plugins, sadly. Take a peek here https://github.com/GeyserMC/Geyser/issues/2355. I did some testing and found out that EssentialsX's /invsee <player> command does work when I assign "essentials.invsee.preventmodify" (prevents picking/moving items in menu for everyone, even op) permission for the player. Well, I already started implementing alternative chat-based commands for bedrock players, that should do the work. Still it would be a great fix, if you come up with something, maybe for future stuff, idk... anyway Thanks for reply!

Update: Tested it on my phone today, and for some reason, all gui menus worked flawlessly - when I tried to click on the item and move it into another slot(to trigger a click same as on java edition) everything worked and the items I clicked on quickly updated/returned back after click. I don't know how, maybe it's related to a better ping, or server having less active processes, but I'm very sure the tps was stable 20 for all tests I did with this. Maybe I had some other active mycmd command or something, I will do more tests if I come up with something. I know that puting $delay$<20> in runcmd pauses all mycmd commands, might be something to do with tihs, but I might be wrong, since I don't use delay that much.