Original Title: It appears when you press the button of slot without item and scroll to slot where Item located (Trident).

You need firstly activate attack of trident then U need to scroll to another (empty) slot item, then just press button of slot with trident or bow without unpressing right mouse button.

Edit by KILLERBEENL83
Steps to reproduce:
Creative Windows 10, kb and mouse, verification build 1.6.0, 1.7.0.5 (fresh install).
1. Have a trident in inventory slot 1 and a empty hand in slot 2.
2. Select slot 1 with the trident (keyboard numbers are used)
3. Press and hold right mouse button and wait for the trident charging animation to finish
4. Keep holding right mouse button and press 2 (slot 2)
5. Keep holding right mouse and press 1 (slot 1)
6. Release right mouse button

Additional notes:
The issue was also reproduced on Xbox One S running 1.6.0 using a different KB and mouse. And on a laptop (1.6.0) with a different mouse and keyboard. Possibly this issue might not be reproducible on all hardware or versions.

Video attached:
MCPE-37581.mp4