I can confirm the issue with Linux Mint21 and OpenSUSE Tumbleweed (both 64bit and gtk2), and Linux Mint 19 (32 bit, gtk2; BUT NOT gtk3)
Please post a bug report and attach your demo.
Some observations (please mention them in the bug report):
- So far, I only found the Label to be the only control which causes this.
- I can switch pages by code, i.e. PageControl1.ActivePageIndex := 1 is fine. It seems to be the click which causes the problem.
- When the Labels's AutoSize is false, the bug is gone.
- It is essential that the PageControl is inside a GroupBox; a PageControl directly in the form, or in a Panel, does not show the issue.
As a workaround, you can use a TStaticText.