not sure why the modal is not centering on the page.
We are using legacy semantic unfortunately and cannot upgrade. 0.x

additionally, if the modal is too long, it is hidden under the browser window

Same here, modals with a large .ui.fluid.image (or .ui.large.image) shows like your second screen. Strangely it happens only on the first opening (if i reopen the modal it shows normally).
in javascript, add this setting to the modal:
observeChanges: true
And, if u are using ANGULAR... maybe a timeout for modal (100ms is enough) will fix too...
Yes, i confirm that angular was the problem. A timeout fixed it ;)
@ps1dr3x @Keten thank you, I was using angular as well. I managed to fix using a .refresh...and will use your solutions instead.
thank you that setting is the solution.
in jquery the modal position is not centering when i opened first time later on its working fine any solution for that
Setting solution worked out for me, thanks!
Most helpful comment
in javascript, add this setting to the modal:
observeChanges: true
And, if u are using ANGULAR... maybe a timeout for modal (100ms is enough) will fix too...