My kitten walked on my keyboard...

Somehow, my mind went off on a tangent and assumed that you were using a trackpad, either external, or on a laptop — despite my seeing the mac mini in your profile, and earlier reference to mouse. Sorry.

Here is what I think is causing the trigger of Dashboard. In System Preferences : Mission Control, one can assign a middle, or secondary mouse click to opening Dashboard. In this panel, look down to where you see Show Dashboard, and it probably is configured with F12 by default. In the next column over, is the mouse configuration. Set it to '-'.

Here is my settings on High Sierra 10.13.3.

Ralphie entered a code....cats are quick like that 🐾

This is what I found regarding a message appearing to the right in any manner:

Mail automatically checks your email accounts periodically for new mail. To manually check for email, click the Get Mail

button. To view an email message, select it from the list of messages on the left side of the Mail window. The message body appears on the right side of the window.

Otherwise, I'd check my mouse settings in system preferences. See if you have your secondary button set to right-click and all settings are what you prefer.

Take care

You're welcome.

I saw a post about a black screen image in mail late last night that's similar to your issue and in which the right side drops from the screen. It was something in the display setting I believe. Do a Google search.

Sorry I couldn't help more. Maybe someone else will be able to assist

You have opened the Dashboard with no applications shown. This is not the same as the macOS Desktop. In the lower right corner, that right arrow will take you right back to the macOS Desktop.

I actually removed the Dashboard icon from the Dock to avoid accidental launches of it. If you hadn't removed it, and your pointer was resting on it when you cat strolled across your Mac, any paw that touched the trackpad would have launched Dashboard to the screen that you show above.

In System Preferences : Trackpad : More Gestures, if you have Swipe between full-screen apps selected, you can use four fingers swiping right to reveal the Dashboard desktop that you have shown. Another four finger swipe to the left will reveal the macOS Desktop. I have this selection disabled.

Here…

When I right-click my wireless mouse, dashboard comes up, making the right-click functions not work. It used to work so I don't know what has changed. Any ideas?

Try this link?

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Here is a test. Open AOL or Gmail. On that application icon in the Dock, right-click, and then select Show All Windows from the open app dock menu item. Is this the effect you are experiencing?