The thin line L indicates that the computer has accepted a left click.

The image remains on the screen until the mouse cursor is moved out of the test area and back in. When the cursor re-enters, the test area is cleared and one can perform a new test.

to test a double click:

Move the mouse cursor into the grey rectangle and produce two mouse clicks in rapid succession.

There are two requirements for a valid double-click in Windows:

One:  The second click must follow the first one within a certain time.

Two:  The mouse cursor is not moved during the time between the two clicks.

The position of the dashed line in the test area shows the Windows double-click time. The second click must start before the time line reaches the dashed line. Otherwise Windows will ignore the double-click and treat the action as two separate clicks. The setting for double-click time can only be changed in Windows: control panel - hardware - devices - mouse.

The start of the second click (where the green time bar rises again) lies well before the dashed line. The red line D indicates the moment that Windows has accepted the two clicks as a double-click.

The short grey marks in this image, indicate mouse movements.