Prevent window auto-resize on screen change

If your windows are jumping from about 90% size to 150% size whenever you move a MacBook Pro between displays, this is usually caused by macOS Tahoe's window management features trying to "help" by restoring, tiling, or reflowing windows based on the new screen geometry. 

There doesn't appear to be a dedicated "don't resize windows when display changes" switch in Tahoe 26.5.1. 

Tile windows on Mac - Apple Support

A - Turn off window tiling

Open System Settings >>Desktop & Dock

• • Under Windows, disable "Drag windows to left or right edge of screen to tile", "Drag windows to menu bar to fill screen" and "Hold ⌥ key while dragging windows to tile" 

B- Disable Stage Manager

System Settings >> Desktop & Dock >> Stage Manager, Turn it off completely.

Stage Manager is known to resize and reposition windows automatically. 

C - Check Display Scaling

If one monitor uses a different scaling setting than the other (for example "More Space" on one display and "Default" on another), macOS may resize windows to fit the available workspace when switching displays.

Go to System Settings >>Displays and compare the scaling settings for both screens.

D - Turn on "Displays have separate Spaces"

System Settings >> Desktop & Dock >>Mission Control

Enable Displays have separate Spaces

Log out/in if prompted.

This often reduces aggressive window rearrangement when connecting or disconnecting displays.

[What follows is only my interpretation, based on experience, but I could not find any documentation (for or against).]

Windows can change size due to the two screens having different resolutions.

For example, the internal display may be set at 1512*982 (half the physical pixel dimensions in each axis, as most "retina" resolutions are). A window there is filling the screen (just under the menu bar, not in full screen mode).

An external display attached may be 1920*1080. Moving the window to the other screen, the overall size of the window changes - but, crucially, the size of the TEXT remains the same. This is because the external display, although nominally it has more pixels, in reality does not (as the retina display has physically 3024*1964 pixels).

Basically, what I am saying is that the size of a window, per se, is not the most relevant, but its contents, and consistent display thereof, are.

Note that once that window is moved back, it regains the exact size it had before.

I am trying to understand if the update to Tahoe 26.5.1 is or is not the cause of the screen jumping and resizing now. It only began on our desktop after the update and it doesn't resize or jump all the time, but is unsettling (thought it was a bug or such). I am thankful for the instructions but would like to understand the cause of the changes after the update. Not to be Pollyanna about it, but shouldn't an update make it better and not cause more issues so that the user needs to use any of the A, B, C and D you listed?

I am grateful for response(s).

Your reaction is reasonable.

In general, a software update should improve things, fix bugs, close security issues, improve compatibility, or add features.

When a new issue appears immediately after an update, it is natural to ask whether the update is responsible.

Regarding macOS Tahoe 26.5.1 

The fact that, the screen jumping / resizing began only after the update and It did not occur previously,

The behaviour is intermittent rather than constant, does suggest a possible connection.

However, it does not necessarily mean the update itself is defective.

On this, I would concur on my colleague's interpretation