Bisecting gives this commit range:
https://chromium.googlesource.com/chromium/src/+log/46d51e23c50af2dcfa549e5f36f52464d36487ce..de8970759a44d64902b6541ab797d2d4bdf78470

The first revision with this bug is https://chromium.googlesource.com/chromium/src/+/de8970759a44d64902b6541ab797d2d4bdf78470

These icons are ToolbarActionViews which are MenuButtons.

I'm guessing OnMouseEntered() is not being called when the mouse enters from the top? 

Regression range: https://chromium.googlesource.com/chromium/src/+log/ad21cca2c4c2403df81307f5a0540c8bbff17b42..524b14d53b65b7f27872ef3586d5a2c58d265385

cyan@: Can you please take a look at this regression? 

A further issue case is, if you have two item in the main menu and drag one of the items to another position (note that the mouse cursor is still on that item). Now if you move the mouse cursor within this item, the hover effect should appear again. But it doesn't :( Only if you move out of the item and move then back over it, the hover appears again.

Thanks in advance :)

This issue has an owner, a component and a priority, but is still listed as untriaged or unconfirmed. By definition, this bug is triaged. Changing status to "assigned". Please reach out to me if you disagree with how I've done this.