App menu moves left when third party app hides the app menu icon #5787

Environment

• BetterDisplay 5.0.4 pre-release (build 53176). Same behaviour on 4.3.6.
• macOS 26.6.2 (25G83), MacBook Pro 16" M3 Max (Mac15,8), 64 GB, lid closed.
• Displays: two HKC N07 4K panels over Thunderbolt at 2304x1296 HiDPI, left one "N07 2" (displayID 3) at (-2304, 0), right one "N07 1" (displayID 2) at (0, 0) and set as main; Elgato Prompter XL (DisplayLink, 1920x1080) at (2304, 0). One BetterDisplay virtual screen, disconnected. Displays have separate Spaces is on.
• DisplayLink auto-discovery is off (workaround for the DisplayLink 14 NaN crash, since fixed by DisplayLink 16.2.39).

Steps

1. Click the BetterDisplay menu bar icon on the right monitor.
2. The app menu opens under the icon as expected.
3. Roughly one second later, without any input, the menu moves left to about the middle of that monitor.
4. Roughly one second after that it moves left again, this time onto the left monitor, where it stays until closed.

The same happens when the icon is clicked on the left monitor. The menu opens under the icon, then steps left in the same two moves and ends pinned to the left edge of that monitor. In both cases the menu stays open and works. It is only its position that changes.

Screen recording

A 28 second recording taken on the left monitor shows the two steps clearly. Attached in a comment below as a 1 fps, silent version of that recording: menu under the icon at the right, then at roughly 60 per cent of the width, then flush with the left edge. The pattern repeats identically on every open.

Ruled out

• Not focus stealing. Reproduced with every other app quit, including a dictation utility that had been frontmost, and with no windows open on the left monitor.
• Not the PIP window. No PIP window is open. (Release notes for 4.3.3 mention a similar jump when a PIP window was on another display.)
• Not menu height. Reproduced after collapsing three of the six sections so the menu no longer reaches the bottom of the screen.
• Not the version. Reproduced on 4.3.6 and on 5.0.4.
• Not the virtual screen. It is disconnected.
• Configuration Protection and any main display override are unset. macOS main display is the right N07.

Expectation

The menu stays under the icon that was clicked.

Extra observations that may help

• BetterDisplay keeps a 1x1 pixel window at (0, 1295), the bottom left corner of the main display, exactly on the seam with the left monitor. If the app menu is anchored or screen-matched through that window, a point on the seam being attributed to the left screen would explain the leftward drift.
• The two moves are about a second apart, which matches the menu's periodic refresh rather than a user event.

Happy to provide the recording, a console log capture during an open, or to test a build.