Mirroring displays breaks the screen zoom accessibility feature

[Don Pezet]

I have found that in Ubuntu Linux if I mirror my laptop's display to an external monitor the screen zoom feature no longer functions. I have observed this issue under clean installs of Ubuntu Desktop 16.04.2 and 16.10 on a System76 Lemur laptop with HDMI output.

These are the steps I used to recreate the problem. The steps are the same for 16.04 and 16.10.

1) I used the Unity Tweak tool (Unity Tweak Tool -> General) to assign keyboard shortcuts to zoom in/out on my screen. I assigned Ctrl-Shift-Z for zoom in and Ctrl-Shift-X for zoom out.
2) I tested the shortcuts by pressing the assigned key combinations. That functionality worked as expected.
3) I connected an external display to my HDMI port. The display immediately activated with an extended desktop.
4) I repeated the test from step 2 and zoom in/out worked as expected.
5) I enabled display mirroring in the system settings (Settings -> Displays -> Mirror Displays). The displays mirrored as expected.
6) I exited the settings screen and repeated the test from step 2. Now, when pressing the zoom in combination, the mouse cursor disappears and the screen does not zoom in. Pressing the zoom out combination causes the mouse cursor to reappear.
7) I then disabled screen mirroring in the settings. This returned me to an extended desktop.
8) I then repeated the test from step 2 and the zoom in/out functionality worked as expected.