Issue Summary

When opening a card back in Trello, focus is automatically directed to the first interactive element (the list dropdown / "Move" action). As a result, pressing the Spacebar triggers the focused button/dialog instead of assigning or unassigning the user (Join/Leave card).
Because the Spacebar serves multiple standard purposes in browsers and triggers interactive elements, users can no longer seamlessly join/leave an open card using just the Spacebar without first clicking away or taking extra steps, interrupting rapid keyboard navigation.

Suggested Solutions

1)Allow users to customize key mappings/shortcuts (e.g., assigning a key combination or a non-conflicting key other than Space to Join/Leave cards).

2)Provide an option in Trello preferences to adjust shortcut behaviors for power users while maintaining accessibility standards by default.

3)Introduce a dedicated, non-interfering modifier shortcut (e.g., Shift + Space or Alt + Space) specifically for joining/leaving while on the card back.

Workaround

Use m + Enter (or m + type your name + Enter) while the card back is open to quickly assign/unassign yourself via the Members menu without using the mouse.

Click on an empty/blank area of the card back background .

Hover over the card on the board view (card front)  and press Space before opening the card.