I am using Big Sur. Inside the Messages app when I click the green circle in the upper left hand corner while holding the Option key the green icon will display a black plus (+) sign and the window will maximize to take up the whole screen except the dock and the menu bar. I call this Maximizing the window.
This same functionality does not work in Safari. In Safari I use the same key combination and the green circle shows a plus (+) symbol when the mouse hovers over the icon, however when I click the Safari window does not expand all the way. It only uses about 60% of the horizontal area of the screen instead of the full 100% of the horizontal area.
Is there a way to correct this? It is very annoying to have to manually drag the edges of Safari to fill up the screen.