MacBook Pro (15-inch, 2017)
MacOS High Sierra 10.13.6 (17G65)
Processor: 3.1GHz Intel Core i7
Memory: 16 GB 2133 MHz LPDDR3
Graphics: Intel HD Graphics 630 1536 MB
Slack
Version 3.3.1 (Direct Download)
Karabiner-Elements 12.1.0
Problem:
With Slack, and Karabiner-Elements running, I can NOT use Shift + Esc to 'Mark All as Read'. I have tested the Shift key and the Escape key in various other applications/scenarios, and and confirm they're working fine independently. I have captured the keypress events in the Karabiner Event Viewer:
Generic Shift + Esc:
eventType:key_down code:0xe1 name:left_shift misc:
eventType:key_down code:0x29 name:escape misc:
eventType:key_up code:0x29 name:escape misc:
eventType:key_up code:0xe1 name:left_shift misc:
Shift + Esc in Slack:
eventType:key_down code:0xe1 name:left_shift misc:
eventType:key_down code:0x29 name:escape misc:
eventType:key_up code:0x29 name:escape misc:
eventType:key_up code:0xe1 name:left_shift misc:
Karabiner-Elements appears to be able to capture the keystrokes, but it does not seem to be making its way into Slack. I am able to consistently reproduce this problem, and can confirm that Shift + Esc does work when Karabiner-Elements is not running.
Please let me know if there is any additional information I can provide to troubleshoot this issue.
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
I have the exact same problem. Shift+ESC doesn't work as a shortcut as soon as karabiner is used.
Yeah unfortunately this bug was marked stale. In the event where I really need to mark all as read, I just close out Karabiner, do the Shift+ESC, and open Karabiner again.
Is there any way to reopen this?
This used not to be the case and I thought Slack was at fault initially.
I have the same problem on macOS. But my co-worker can use ⇧ + Esc on Slack with Karabiner-Elements on macOS. But his configurations are bitty large, so I could not determine what cofigurations are needed to avoid this problem.
I could fix the problem by enabling "Modify events from this device" to the disabled keyboard.
This keyboard have been displayed as No product name (No manufacturer name) on Karabiner-Elements Preferences > Devices.

I confirmed the environment.
Thank you @Kuniwak, I had this problem in other apps using shift and function keys, it solved that as well.
Thanks it solved my issue with the shortcut to force quit applications.
Thanks it worked
The suggestion in the @Kuniwak's comment worked for me, too. I think this issue should be reopened as it may be a bug in Karabiner-Elements or at least very counter-intuitive behavior.
Most helpful comment
I could fix the problem by enabling "Modify events from this device" to the disabled keyboard.
This keyboard have been displayed as
No product name (No manufacturer name)on Karabiner-Elements Preferences > Devices.I confirmed the environment.