Edit: Bug tracker https://bugs.chromium.org/p/chromium/issues/detail?id=1061783

Recently there has been a bug where convertible Chromebooks are getting stuck in tablet mode while in the clamshell mode. I know many people who actually need this probably won't read this because searching is weird, but I digress.

The issue happens randomly, but often after the lid is closed. The behavior is consistent with tablet mode if a keyboard is attached, but not a mouse.

Some quick fixes are:

1. Restart the computer either normally or just restart the os using chrome://restart. This gets rid of the issue until it comes back
2. Plug in a USB-mouse (a wireless dongle is enough.) This forces the normal use case since now the os sees a mouse so it won't force tablet navigation. However this still has the "rotation lock" menu item in the quick settings and the device might rotate, but I haven't experienced any issue like this myself.
3. E*: Enable the flag chrome://flags/#ash-debug-shortcuts, this allows you to toggle between tablet and clamshell modes with the keyboard shortcut Ctrl Alt Shift T.

E* =edit