This strange bug with Extend has started happening today. This is on Ubuntu 20.04, and only seems to happen in Chrome. I have tried using Firefox, Geany and Terminal and found no problems with any of those.
The problem is this.
Pressing Ext-t produces a "t" character. Similarly, pressing Ext-s produces an "s". Other Extend keys are working OK, for example arrow keys, copy/paste, etc. So the bug seems to be affecting the Extend modifiers only.
However, the modifier itself is still working, i.e Ext-t is still registering Ctrl, despite the "t" appearing.
So this sequence: hold Extend, hold t, tap n, release t, release Extend.
produces this result: "t" appears on screen, then cursor (correctly) moves back one word.
I'm not using kmonad or anything else that could be interfering AFAIK. I tried disabling all extensions in Chrome, also no effect.
I did do an OS update yesterday so maybe Chrome got updated and now has this bug? Chrome version is 86.0.4240.75.
Obviously this is annoying as it affects any typing you do it text boxes on Chrome. The workaround is to use Firefox.
I'm assuming no-one else has encountered this yet, but I'm putting it here in case anyone else experiences it.