For many buttons on the tools bar you can access the corresponding setting by a right click on the button.
When “Distance measurement” is down, the mouse left click behavior change.
Click on the first point, without releasing the button move the cursor to the second point and release the button.
The distance, position angle, RA and DEC offset is displayed on the left of the status bar.
The first and second point can be any position even without an object. But if an object is identified near the cursor its center position is used instead of the mouse cursor position. In this last case the identification label is show near the object.
Click the “Distance measurement” button again to return to normal cursor use.