MacOS desktop client doesn't open plugin links in browser

[svenssonaxel]

Steps to reproduce:

1. In the MacOS Mattermost desktop client, connect to the server at community.mattermost.com. We will call this instance of Mattermost Window 1.
2. Go to direct message and select yourself.
3. Send a message containing the following: [Good link](https://github.com/) and then [Bad link](https://community.mattermost.com/plugins/dummy.plugin.name/nonexistent-page?variable=value). A message appears containing two links.
4. Turn on full screen mode for Mattermost by pressing Cmd+Ctrl+F, or by clicking the green button in the top-left corner of the window.
5. Click on "Good link". A normal browser window appears, i.e. not in full-screen mode, showing Github front page. Mattermost client remains accessible in full screen mode by moving to the application's full screen window.
6. Exit the browser window to return to Mattermost client, still in full screen mode.
7. Click on "Bad link".

Expected:
The link titled "Bad link" is opened in a new browser window, similarly to step 5, but showing "404 page not found".

Actual:
No browser window is opened. Instead, the link is opened in a new Mattermost full-screen window (which we shall call Window 2). Window 2 does show "404 page not found" as expected, however the green button for toggling full screen mode disappears. While Window 2 is in full screen mode, Window 1 is not visible or accessible. However, a separate full screen window of Mattermost that appears entirely black (which we shall call Window 3) is visible in and accessible through Mission Control.

The full screen mode for Window 2 can be exited either through Mission Control by using the button which appears in the top left corner of the corresponding window thumbnail while hovering over it, or by pressing Cmd+Ctrl+F. This moves you to the desktop, where Window 1 becomes visible and accessible, and is no longer in full screen mode. Window 3, still fully black and in full screen mode, remains accessible and visible through Mission Control by clicking on its thumbnail. When Window 2 is no longer in full screen mode, it can be closed by pressing the now-visible red button on its top-left corner. Additionally, Window 1 is sized slightly larger than the desktop, leaving some of it inaccessible outside of the visible screen. Dragging to move Window 1 does not solve this, as it automatically snaps back to its original position partially outside of the screen upon release. Pressing Cmd+Ctrl+F while Window 1 is selected in this state will not cause it to enter full screen mode, but rather resizes it to a size that fits on the desktop. After this, Window 3 remains entirely black, visible and accessible through Mission Control. If the user navigates to Window 3 through Mission Control, full screen mode cannot be exited by pressing Cmd+Ctrl+F. Moving to Mission Control and using the button that appears upon hovering on the top left corner of the window's thumbnail to exit full screen mode for Window 3 causes the window to close.

Note: The web app does exhibit the expected behavior by opening the link in a new tab. This is tested with Firefox 131.0.2 under X11 on Linux.

Version info:
MacOS 15.3.1
Mattermost MacOS desktop client version 5.11.1 (135901780382)

[devinbinnie]

So the popout is actually done for a specific reason, as many of our integration plugins (ie. GitHub, JIRA, GitHub) require an external sign-in, and we make sure that's done in a separate window. We support the external sign-in case for Mattermost itself by passing a token, but this was never extended to plugins as it would require a lot of plugins to update if we changed the API, which would cause some compatibility issues.

I'm not sure if we'll be able to change this any time soon, however they are some changes coming around popout windows that we might be able to take into account. As for the difficulty around fullscreen mode, until we make those popout window changes the workaround of manually pressing Cmd+F seems to work for me.

Is there a specific workflow that this is breaking for you? Or is it more of an annoyance?