If you don't need the tabs and use the PageControl only to arrange controls in several "layers" it's better to use a TNotebook rather than a Pagecontrol.
Not usre about LCL, but in Delphi this legacy component had more and more rendring problems as time went buy. Eventually i had to switch to TPageControl wit hhidden tabs.
But maybe in LCL it is different.
Thanks for head-ups on Win11 - it is insane if true...
Nicole, another trick would be - to place many TPanel on the form, set there Bevels and Borders to None. Then to switch them all to alClient, so they are on top of one another. This is similar to what Notepad does behind the curtails.
Then, how to switch panels? In code - "Panel10.BringToFront", and in IDE there would be BringToFront r-click menu :-)