The computer can only open a pre opened tab in a new window if you pressed the relevant keys or selected the relevant menu items or depressed a mouse left button whilst moving the mouse pointer. The computer is acting upon info it gets from the keyboard or mouse and only you have control of those items.

re : but that mail tab will have switched to a different mailbox. Any new window will also have a tab which displays the Inbox of the default email account, so that is normal and nothing to be worried about.

re :What seems to happen is that the new window will contain the tab I was working with (address book, settings, etc.)

Lets say you have opened 'Address Book' in a new tab and you have focus on that tab. You are doing something almost like an unknown habit which is causing that same tab to open in a new window. But what are you doing is the question. Sadly I cannot sit next to you and observe, but I have been around a while and seen many cases of this nature.

How to get that tab to open in a new window and display the tab 'Address Book' in a tab plus it will always show the Folder Pane and focus on the default account Inbox.

• Right click on the 'Address Book Tab and select 'Move to new window'
• Right click on the 'Address Book Tab and press 'W' key it will open in a new window
• Press the key which displays drop down options (as if you had performed a right click - it is usually located next to the 'Ctrl' key on the right side of keyboard) and press 'W' key.

Actually the number of people who seem to accidentally rest their hands on the bottom keys on a keyboard is a lot more common than you imagine. So if you pressed 'W' and issue occurred then it's highly likely you had part of your hand resting on the drop down key at same time.

But it's also possible to do it without pressing any keys. This method is also quite common because people go to move the mouse and auto press the left button without really knowing this is something they had done and it causes no end of confusion.

• Click on mouse whilst mouse pointer is hovering over the tab title and drag to beyond any window edge.

So in effect you think you are just moving the mouse to a desired spot and do not realise you are doing more by depressing that button. I've even seen people absent mindedly move a mouse whilst reading a list and wonder what on earth happened.

@Toad-Hall, I understand what you're saying, but I can assure you that I am not accidentally performing any mouse or keyboard actions that is consistent with what is happening as intended Thunderbird behavior. This is definitely a bug, not user error. But after reading through your description, I can better describe what exactly is happening and what the triggers for it are.

What's happening is indeed functionally equivalent to either:

• Right-clicking a tab and picking "Move to New Window"
• Dragging a tab out of the current window to open it in a new window

This is just speculation, but I suspect that Thunderbird is erroneously misinterpreting my mouse actions as the latter. And the reason is that I've found a way to trigger this bug with 100% consistency, and it involves perform clicks and mouse cursor movements (but no mouse drags) in a particular way.

Here is a video of me triggering the bug: https://nextcloud.briankendall.net/s/BHzWncB3RGxPFJe

As you can see in the video, it displays precisely when I'm pressing the mouse down, and you can see there is no action in there that can explain what's happening, other than it being a bug. I am also not using the keyboard at all in that video.

So that others can try, here is the precise method I found to reproduce the bug:

1. Open Settings in a new tab, so that there are now two tabs: Mail and Settings 2. Scroll down to the bottom of the Settings tab or enter "config editor" in its search field to reveal the "Config Editor..." button 3. Click "Config Editor..." to open Advanced Settings in a tab to the right of Settings 4. Click the Settings tab to switch to it 5. Click the Advanced Settings tab to switch to it 6. Move the mouse cursor in an upward direction without pressing any mouse buttons

The moment the mouse cursor moves upward, the Advanced Settings tab will suddenly jump into its own window. This happens 100% of the time for me, and involves no keystrokes, and no mouse drags. All clicks are only mouse down and up events, and I've confirmed this by watching the list of events that are passing through my mac.

I think this bug is also related to having a toolbar that contains buttons whose style is set to "Icons above text". At the very least I can trigger it much more easily when switching between tabs where one tab has a toolbar containing icons only, and the other has icons and labels below the icons. This causes the tab bar to change position every time I switch between the tabs, which might be part of what's triggering the bug. So if you have trouble reproducing the issue with the above steps, I recommend modifying the Settings tab's toolbar so that it contains at least one button, and setting its button style to "Icons above text", so that when you switch between Settings and Advanced Settings, the tab bar's position shifts significantly.

I don't suppose there's any way to work around this bug, is there? Either way, it looks like me or someone else will need to file a bug report and hope it gets fixed in the near future.

edit: I submitted a bug report. I'll keep the question around in case anyone can find a workaround for this.

@Toad-Hall, I understand what you're saying, but I can assure you that I am not accidentally performing any mouse or keyboard actions that is consistent with what is happening as intended Thunderbird behavior. This is definitely a bug, not user error. But after reading through your description, I can better describe what exactly is happening and what the triggers for it are.

What's happening is indeed functionally equivalent to either:

• Right-clicking a tab and picking "Move to New Window"
• Dragging a tab out of the current window to open it in a new window

This is just speculation, but I suspect that Thunderbird is erroneously misinterpreting my mouse actions as the latter. And the reason is that I've found a way to trigger this bug with 100% consistency, and it involves perform clicks and mouse cursor movements (but no mouse drags) in a particular way.

Here is a video of me triggering the bug: https://nextcloud.briankendall.net/s/BHzWncB3RGxPFJe

As you can see in the video, it displays precisely when I'm pressing the mouse down, and you can see there is no action in there that can explain what's happening, other than it being a bug. I am also not using the keyboard at all in that video.

So that others can try, here is the precise method I found to reproduce the bug:

1. Open Settings in a new tab, so that there are now two tabs: Mail and Settings 2. Scroll down to the bottom of the Settings tab or enter "config editor" in its search field to reveal the "Config Editor..." button 3. Click "Config Editor..." to open Advanced Settings in a tab to the right of Settings 4. Click the Settings tab to switch to it 5. Click the Advanced Settings tab to switch to it 6. Move the mouse cursor in an upward direction without pressing any mouse buttons

The moment the mouse cursor moves upward, the Advanced Settings tab will suddenly jump into its own window. This happens 100% of the time for me, and involves no keystrokes, and no mouse drags. All clicks are only mouse down and up events, and I've confirmed this by watching the list of events that are passing through my mac.

I think this bug is also related to having a toolbar that contains buttons whose style is set to "Icons above text". At the very least I can trigger it much more easily when switching between tabs where one tab has a toolbar containing icons only, and the other has icons and labels below the icons. This causes the tab bar to change position every time I switch between the tabs, which might be part of what's triggering the bug. So if you have trouble reproducing the issue with the above steps, I recommend modifying the Settings tab's toolbar so that it contains at least one button, and setting its button style to "Icons above text", so that when you switch between Settings and Advanced Settings, the tab bar's position shifts significantly.

I don't suppose there's any way to work around this bug, is there? Either way, it looks like me or someone else will need to file a bug report and hope it gets fixed in the near future.

edit: I submitted a bug report. I'll keep the question around in case anyone can find a workaround for this.

Thanks for your feedback and the vidoes really helped explain things. The video shows you are using a MAC, but this is not MAC specific.

I've been retesting this after seeing your videos and I've managed to replicate. But in all cases I must still be grabbing the tab even if for a fleeting moment as I initially start to move the mouse. In other words I've not fully released the mouse before I move it. If doing this quickly, there is barely a fleeting moment between movement and release of left click, but if it's still depressed for a millisecond and I start to move mouse then window opens. So it's rather sensitive.

But if the code is designed to open a new window if you use left click hold down and move downwards on a tab then it's working perfectly. That is a decision the developers will need to consider.

Just wondering.... On my Windows OS 'Mouse Settings', it's possible to set up something called 'ClickLock'.

Depending upon the length of the left click being depressed, it can auto switch on 'ClickLock' which means I've grabbed the item I clicked on so I can move without the need for keeping a left click hold down in order to grab. A second left click releases it.

This annoyed me too much causing all sorts of accidental movements of stuff, so I disabled it. Check to see if you have something similar.

Check to see if you have something similar.

Nope, no click lock. I have a tool that lets me monitor all of the mouse events going through the system (I am a macOS developer) and I can see that, when the bug triggers, it's a "mouse moved" event that's passing through and being received by Thunderbird, not "left mouse dragged".

I've seen this issue posted elsewhere - did you also post this in reddit or bugzilla?

I've seen this issue posted elsewhere - did you also post this in reddit or bugzilla?

I made a bug report in bugzilla after posting this thread. I don't believe I've posted about it elsewhere.

Yes. It has been reported in Bugzilla.

I've done loads of testing in Windows OS and I can only reproduce if I have the left click held down when I move downwards - even if it is only for a moment at initial start of movement.

But it looks like this bug is very real and maybe MAC OS specific as someone else has managed to reproduce in a MAC. Bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1885975