A document from 1981 reveals the reason!
The first mouse cursor was indeed vertical
""There seems to be a reason why mouse cursor is slightly tilted which was found in a document from 1981.
Have you ever wondered why mouse cursor on your computer is slightly tilted and not straight? Why isn’t the cursor positioned vertically which would be a more logical choice? Because that would be more convenient for selecting text and other tasks.
An expert at StackExchange, who is quite familiar with the history of computer technology, explained the reason by referring to a document that is more than 30 years old (from 1981). He confirmed that Douglas Engelbart, who invented the mouse, in fact portrayed the arrow cursor vertically as the most logical selection option.
However, the graphical interface of the operating system in practice was first implemented by Xerox, namely – its research and development unit Palo Alto Research Center (PARC). In particular, the cursor was described in a document from 1981 , which lists the characteristics of XEROX PARC, the first ever Graphical User Interface (GUI) in history.
When designing this machine, it was practically impossible to design a vertical mouse arrow of small size (that could easily be seen by users) due to low resolution displays. That’s when they decided not to increase the size of the pointer, but instead turn it slightly so the left side of it was vertical and the right one tilted under 45°.
More than 30 years have passed since then, high-resolution displays were invented, but the tradition of portraying the mouse cursor tilted by 45° preserved."
Source
The first mouse cursor was indeed vertical
""There seems to be a reason why mouse cursor is slightly tilted which was found in a document from 1981.
Have you ever wondered why mouse cursor on your computer is slightly tilted and not straight? Why isn’t the cursor positioned vertically which would be a more logical choice? Because that would be more convenient for selecting text and other tasks.
An expert at StackExchange, who is quite familiar with the history of computer technology, explained the reason by referring to a document that is more than 30 years old (from 1981). He confirmed that Douglas Engelbart, who invented the mouse, in fact portrayed the arrow cursor vertically as the most logical selection option.
However, the graphical interface of the operating system in practice was first implemented by Xerox, namely – its research and development unit Palo Alto Research Center (PARC). In particular, the cursor was described in a document from 1981 , which lists the characteristics of XEROX PARC, the first ever Graphical User Interface (GUI) in history.
More than 30 years have passed since then, high-resolution displays were invented, but the tradition of portraying the mouse cursor tilted by 45° preserved."
Source
