My windows keep expanding to near-full screen upon first touching the trackpad.

Sounds like the new tiling feature. Try turning it off:

System Settings -> Desktop & Dock -> Tile by dragging windws to screen edges OFF

You can still do it while holding the Alt Option key and dragging windows. They will snap full screen, left, right, and top, bottom.

Never used third party tilers except for the LG one for a minute and then removed. This issue is very annoying. every time a new Safari window opens or I click on an email it goes full screen. I have previously (today) sent apple feedback about this. I checked the setting you suggested above. And for many versions of OS, including Sequoia, the double-click window titlebar has been turned off. Never had a need for it it was an annoyance to me as well years ago.

I just noticed, don't recall seeing this before, an option to drag windows to menu bar to fill screen. While that description doesn't fit my issue turning it off does fix it. To be clear I just click mail in the dock and it opens as I had it set last and then immediately expands. No me touching it or moving it.

-amend- it does however explain when sometimes when I move a window to the edge it goes fullscreen since I must be sliding it along the menu bar to the edge. that no longer happens too. Ahh peace back to my world. Thank you for your suggestions!

I found that I also had to turn off "Drag windows to menu bar to fill screen" directly below "Drag windows to screen edges to tile." Now that they're both off, the windows have stopped automatically expanding when I move them.

There are a number of third-party window manager/tiling apps for macOS. Make sure they are not installed. Also, make sure you are running Sequoia 15.1.1.

I don't have those two "Drag windows to ..." settings. But the "Double-click a window's title bar to" was set to Zoom. I changed that to Do Nothing. Problem solved! Thanks, dialabrain.

Just to add, check the settings for System Settings > Desktop & Dock > Double-click a window's title bar to…

FWIW, if you have a Dell monitor, apparently they have software that conflicts with Sequoia Tiling.

I turned this off some time ago and I have the same issue where some apps automatically expand to near full screen anyway when I open them.

-amend- I have tried re-toggling the settings. Not change. It's really annoying because I use an ultra wide screen and apps like mail or safari do not need to fill the whole space.

It's an LG monitor and I'm not running any of its support software that does its own tiling. I can't speak for any internal hardware code. But this only started happening in a recent update to Sequoia. Also if I move an app to the edge of the screen it will go full screen sometimes. I don't get the tile indicators and the setting is turned off. Doesn't do it all the time.

I did this when I first installed Sequoia but with the last update (15.1.1) , it's expanding the windows again even thought "Tile by dragging windows to screen edges" is still turned off. I've toggled the switch on, then off again but not change. FWIW, I'm using an Apple studio 27" display with a Mac Studio. This is really irritating!

Does anyone else have some ideas?

I just queried the same question! Worked for me too.

You're welcome.

@osihara Thank you for posting. I have same problem on LG monitor. This fixed it!!

Thanks for the advice. Worked for me.