Screenshot "Capture an area of the screen" with 2 monitors

I have a monitor (1920x1200) and tv (4096x2160).
My desktop is the monitor and the tv is set to “This screen extends another display and x is 1920”.
When I run “Screenshot” (press the prtsc key) and select “Capture an area of the screen” my desktop is cloned to the 0,0 on the tv.

which lubuntu version?

It is version 19.04.

I assume then that you are using lximage-qt for screenshot (the default in 19.04). Try screengrab, is our (and LXQt) default screenshot utility for 19.10.- sudo apt install screengrab

screengrab works, thanks.

I noticed a problem when capturing an area of the screen the message says to press any key or click the middle or right mouse button.
The scroll bar is supposed to work as my middle mouse button when clicked.
It does nothing after selecting an area of the screen.
Clicking the right mouse button resets the selected area to nothing.

That issue was noted and fixed. Unfortunately it hasn’t been included in the version that is in the Ubuntu archive for 19.04.
The tl;dr instructions should read “Click and drag to draw a rectangle, then double click or press Enter to take a screenshot.”

Great, thanks that works.