This is how it should be - mouse actions trigger on releasing the button, not when pushing it. It allows the user to correct a mistake if they accidentally pressed the button, because they can move the mouse away from the clicked object and the action will not trigger. Many modern self-cooked UI systems seem to forget about this, it's just sad.

Right click is different though, it shouldn't trigger an action in most cases, instead it should bring up a menu. The faster that comes up the better.

On the mac context menus show up on mouse down. This is so you can click, then drag the the menu entry you want, then release the button to select the menu entry. Overall there is only one click, not two. Some people find it faster.

Seeing as we are in a thread on Vivaldi, a spiritual successor to Opera where mouse gestures were quite important for the overall experience, I'll go with "actions on mouse down are just fine".

But didn't Opera gestures always end on mouse-up? That makes it even more important that a context menu only opens on mouse-up, because otherwise you would potentially both open the menu and execute an action at the same time.

This is exactly why some people (including me) prefer this idiom -- right click down to make a context menu appear, nudge the mouse over a few pixels to select e.g. the back button, mouse-up and there you go.

I haven't used Windows in any real capacity since the early days of Windows 7. As I say this is definitely a personal preference thing that i've likely picked up in recent years. It's just mildly annoying to me that every other browser I use has it on mouse press rather than release