Whenever I download something (eg bank or credit card statement, pdf instruction document), my full-screen browser bumps down and becomes slightly smaller - a gap appears at the top of the screen (with whatever is behind it on view)
Any download appears to do it!
I would not expect the brave browser screen to change at all.
I believe this started fairly recently.
Latest version -
Version 1.28.105 Chromium: 92.0.4515.131 (Official Build) (x86_64)
Also latest MacOS Big Sur - 11.4