When I have multiple viewports and I want to change Viewport Properties for a single small viewport, I would normally access "Viewport Properties" pull-down in the upper right corner of the viewport. However, with Blender 2.90 those buttons are not visible when viewport is "too narrow". Is it possible to open that dialog some other way?
All menu bars and panels everywhere in Blender can be scrolled by holding middle mouse button down over the area you want to scroll. Most modern mice have middle button integrated with the mouse wheel so in practice you hover the mouse over the bar that you want to scroll (in the above image the bar with words "View Select Add Object"), press and hold mouse wheel down and move mouse towards left and you can access the end of that bar. The same method works for accessing e.g. all the tabs or tab contents in "too short" Properties Editor.
As far as I know, there's no keyboard accessible way to do this. And it doesn't seem to work with menus because menus have small visible arrows at the top and at the bottom if a menu cannot fit within Blender window.
Thanks to @JohnEason for tip!