I can confirm the bug, exact same behavior here. Upgraded from ubuntu 19.04 and same flickering effect.
* I open the Tweak tool window
* I try to hover one of the left menu's items -> it flickers, I can not manage to click on a menu item sucessfully
* If I resize the window (I have to make it larger so that the menu displays larger), then the menu works as expected and the flickering effect disappears - I have to do it everytime I launch the app, though.
I can confirm the bug, exact same behavior here. Upgraded from ubuntu 19.04 and same flickering effect.
* I open the Tweak tool window
* I try to hover one of the left menu's items -> it flickers, I can not manage to click on a menu item sucessfully
* If I resize the window (I have to make it larger so that the menu displays larger), then the menu works as expected and the flickering effect disappears - I have to do it everytime I launch the app, though.