I just found out I have been lying to guest presenters for years. I am in shock.
When we have a presenter who wants to share their screen in a web conference, I tell them: "We see what you see. Whatever is on your screen shows up for everyone in the audience."
(We'll discount the obvious tricks that the platforms use to hide their own overlaid presenter controls.)
It turns out that my statement is not strictly true. Want to pause here for a moment and see if you can figure out what does not get transmitted to other participants in a screen share session?
I'm warning you… This is so tricky that I didn't realize it in more than 15 years of full time webinar work.
Your mouse cursor. Participants do not see your mouse cursor.
"Wait just one doggone minute, Mr. Smartypants! I know people see my cursor! I verify it with them as I move the pointer around the screen."
Nope. Ain't happening.
What they see is a locally-drawn version of a mouse cursor that matches the position of your mouse. In 99% of all cases, this is functionally identical to seeing your own cursor on your screen. But you can break the illusion and expose the trickery. Just change the size or color of the pointer on your computer. In Windows, you can get to the controls by typing Cursor Size in your Windows Start Menu. You can change your cursor to a big, bold arrow that makes it easy to see on a complex graphic. Or you can change the color. Or you can make it automatically invert from white to black based on what is behind it.
It doesn't matter what you do… On your web conference attendee screens, the cursor shows up as the default tiny white arrow.
I tested this behavior in GoToMeeting, Zoom, Adobe Connect, and Webex. Each of them did the same trick.
It's easy for me to reverse-engineer a probable explanation. Screen sharing is a difficult task for web conferencing software to perform. The presenter's computer has to repeatedly take note of all the pixels on the screen, upload them to the server, bounce them down to all the other participants, and redraw them to create the image. This happens over and over again on a sub-second basis.
(If you are an engineer, forgive me for not going into details about tracking frame to frame differences rather than full image, and compressing pixel groups and the like. Let's keep it high level and conceptually simple.)
As a presenter moves the mouse around the screen, pixels change with incredible speed. At each point in the movement, pixels are changed to the mouse color and changed back again to the underlying image (which itself may be changing) as the mouse moves on. It would place an incredible strain on the screen redrawing mechanism to keep up with that at the high speeds that mouse movement tends to occur.
So some clever web conferencing software engineer figured out that all the software really has to track is the position of the cursor at any moment. Instead of drawing and redrawing the pixels under the mouse as it moves, the conferencing software can transmit the rapidly changing coordinates and let the redisplay routine add the cursor image over whatever is going on with the rest of the screen image.
It's pretty clever. Most people will never notice a difference. But if you happen to fiddle with your cursor image, you need to know that the audience no longer sees what you see. And you are unlikely to pick that up in any of the standard training you get on using your web conferencing product.
Every once in a while, we see a glitch in the matrix that exposes the simulation. This is one of them.