Okay, a few things here.
- To bound multiple monitors on macOS I have found you need to turn off “Displays have separate spaces” in Mission Control System Preferences
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Generally, it is easy to put the larger monitor on the left because the Window is anchored to bottom left coordinate of the multiple monitor canvas. By starting with the larger monitor on the left you can make sure the smaller one on the right s fully covered. If not, the bottom-left of the smaller monitor will be up some pixels in Y on the larger monitor never allowing you to cover its bottom portion.
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Sometimes in cases where monitors are different sizes, the children alignment options are difficult to work with, I recommend manually placing your sub-containers as I did in the attached example for you.
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One last thing, be aware of Display Scaling. If they are the same it is much easier to configure and I usually align to the top of the monitor offsetting the smaller one in the right up in Y. But in my test case for you here, one of my displays was the build in Retina laptop monitor with display scaling 2.0, because of this it was more difficult to figure out the offset for the smaller monitor so it can be easier to align everything to the bottom of all the monitors and make sure your Arrangement in System Preferences > Displays also has them aligned to the bottom. They are all 0 and no offset to worry about now. The Monitors DAT is really your best friend for this stuff.
2_monitors.4.toe (4.1 KB)