Description

I put down two ways to reproduce this problem. One of them is using my add-on SentenceNav, the other one using another my add-on IndentNav. However, this issue is not add-on related per se, it is rather related to how NVDA handles keystrokes containing Alt modifier. I wasn't able to find many built-in NVDA gestures with Alt modifier, therefore using some from my add-ons.
I've been able to track down the source of this bug to commit 9ab0ef8, therefore cc author @michaelDCurran.
My best understaneding of what's happening, is that for long running gestures, if you release Alt modifier before the gesture is done executing, then NVDA decides to send a raw Alt keystroke, or something that is interpreted as raw Alt keystroke to the application, thus triggering the main menu.

Steps to reproduce 1:

1. Install IndentNav add-on: https://github.com/mltony/nvda-indent-nav/releases/latest/download/IndentNav.nvda-addon
2. Download Test file and open it in Notepad: http://elk.duckdns.org/test.txt
3. Position the cursor on the first line of the file.
4. Press NVDA+Alt+DownArrow to find the next line with the same indentation level and release NVDA and Alt modifiers quickly enough - within approximately half a second.

Actual behavior 1:

Main menu is activated - which I suspect due to the fact that raw Alt keystroke has been sent to the application.

Expected behavior 1:

Main menu should not be activated.

Steps to reproduce 2:

1. Install SentenceNav add-on: https://github.com/mltony/nvda-sentence-nav/releases/latest/download/SentenceNav.nvda-addon
2. Open facebook.com in Firefox. This issue is not specific to facebook or Firefox, however, it can be more easily reproduced by slower-to-react environments, which I found facebook in firefox to be.
3. Find some text post in facebook feed, or a text comment.
4. Press Alt+Down or Alt+Up; try to release the Alt key as soon as possible.

Actual behavior 2:

Often times the focus will jump to main menu.

Expected behavior 2:

Main menu should not be activated.

System configuration

NVDA installed/portable/running from source:

Installed/running from installer

NVDA version:

alpha-20739 and later.

Windows version:

Windows 10;
Version 1803 (OS Build 17134.1667)

Name and version of other software in use when reproducing the issue:

Notepad; Firefox 80.0.1 (64-bit).

Other information about your system:

Other questions

Does the issue still occur after restarting your computer?

Yes

Have you tried any other versions of NVDA? If so, please report their behaviors.

Yes, alpha-20718 and prior work just fine.

If addons are disabled, is your problem still occuring?

I need two add-ons to reproduce this, however, I disabled all the other add-ons to make this case clean.

Did you try to run the COM registry fixing tool in NVDA menu / tools?

Yes.