Frequently Asked Questions
- What is Screenleap?
- What software do I need to install in order to share my screen?
- What software do I need to install in order to view someone else's screen?
- Can I view from my tablet and smartphone? Can I share from my tablet or smartphone?
- What technologies are used to create Screenleap?
- Can I install Screenleap locally?
- What is a share code?
- How do I share my screen?
- How do I stop sharing my screen?
- How do I view a screen share?
- How can I share only a particular application or portion of my screen?
- How do I switch the presenter?
- How do I give presenter control to a viewer?
- How many people can I share my screen with at one time?
- What is a Screenleap handle? Why do I want one? How do I get it?
- What is broadcasting?
- Can I broadcast with my personal handle, but still control who gets to see my screen?
- What are the other benefits of creating a Screenleap account?
- I want to integrate Screenleap into my product? Do you have an API?
- Is there a way I can initiate screen shares from Gmail?
- Does Screenleap work with multiple monitors?
- Can I record my screen shares?
- How much does Screenleap cost?
- Do you offer a discount for education or non-profits?
- How many screen shares can I run at the same time with a paid account?
- How can I get a copy of my subscription payment receipt?
- Do you support audio or voice conferencing?
- What is audio conferencing?
- How do I use audio conferencing?
- How much does audio conferencing cost?
- Can I reserve my own conference ID?
- Are there additional features that I can use during a call as the call host?
- Do you have a list of conference call dial-in numbers for different countries?
- What is broadcast audio? How does it work?
Viewer Mouse/Control Sharing
- What is the viewer mouse feature? How does it work?
- What are the limitations for the viewer mouse functionality in the Chrome extension?
- What is control sharing/collaborative screen sharing? How does it work?
- I've shared control with my viewer. Why can't he/she interact with my computer?
Screenleap for iOS
- How do I perform the initial setup to allow for sharing my iOS screen?
- How do I share my screen in landscape mode when using the Screenleap for iOS app?
Screenleap for Mac
- My Mac has MacOS Catalina (10.15) or later. When I share my screen, my viewers only sees the background of my desktop. How can I allow them to see my windows as well?
- How do I grant permission for sharing control to my Screenleap for Mac app?
- Why can't I grant the accessibility permission to the Screenleap for Mac app?
- Are my screen shares secure? What's to prevent unauthorized users from viewing my screen?
- Is my screen data going through Screenleap's servers? Is it stored there?
- I ended my screen share by closing my browser instead of using the "Stop" button. When I click on the screen share link I see the last image of my screen. How can I remove it?
- Are the connections for my screen shares encrypted?
- If I host or view a Screenleap screen share, can the presenter or viewer control, install, or download things from my computer?
- Are my screen shares ever shared with a third party?
- Can I broadcast with my personal handle, but still control who gets to see my screen?
- How does Screenleap compare with other screen sharing tools like WebEx for performance?
- Can I use Screenleap to share a video in real-time?
- How can I improve the performance of my screen share?
- Is there a maximum screen size that's allowed to be shared with Screenleap?
- Which browser has the best performance?
- I'm a non-U.S. user, why is Screenleap so slow?
- I'm trying to share my screen from a VirtualBox instance but my viewers only see a black screen. How do I fix it?
- The Screenleap for Windows app is crashing for me. How can I help track down the problem?
- I accidentally checked "Remember my choice" and then clicked "Do Nothing" when asked to launch the application. How can I get the dialog to appear again?
- When I am sharing my screen, the viewers are seeing the screen zoom in and out constantly. How do I fix the issue?
What is Screenleap?
Screenleap is a screen sharing tool that allows you to quickly share your screen in seconds. Your friends and colleagues can view your screen from any device with a web browser without having to install any software. Screenleap is the easiest way to share your screen with anyone.
What software do I need to install in order to share my screen?
You need to install either the Screenleap for Mac or Screenleap for Windows app in order to share your screen. If you are using the Chrome browser, you can also share your screen by installing the Screenleap Chrome extension.
What software do I need to install in order to view someone else's screen?
Almost any modern web browser will work. The more recent your web browser is, the better the performance. To get the best performance on the desktop, we recommend using Chrome or Firefox. We formally support the following browser versions:
|Browser||Supported Versions||Recommended Version|
|Firefox||3.6 and later||14 and later|
|Internet Explorer||8 and later||10 and later|
|Safari (Mac and iOS only)||5.1 and later||6.0 and later|
Can I view from my tablet and smartphone? Can I share from my tablet or smartphone?
We support viewing from any device with a web browser (including tablets and smartphones). We also support sharing from iPhones/iPads and Android devices. You can install the iOS app from the Apple App Store and the Android app from the Google Play Store.
What technologies are used to create Screenleap?
Our Chrome extension and viewer is built using the latest HTML5 technologies. We also have native apps for Windows and Mac.
Can I install Screenleap locally?
We realize that there are a lot of advantages to being able to install Screenleap locally. We currently do not support local installs of Screenleap but we are considering offering it in the future. Please let us know if you would be interested in such a product.
What is a share code?
A unique share code is created each time you start sharing your screen. A user with the share code will be able to view your screen by either a) going to the Screenleap site and entering the share code or b) by clicking on a link containing the share code.
How do I share my screen?
- Go to http://www.screenleap.com.
- Click "Share your screen now!" button.
- The first time you share your screen, you will need to download and install the Screenleap app, and then accept any prompts allowing Screenleap to run.
- As soon as you start a screen share, you will see a page that offers you many easy ways to give your friends and colleagues
access to your screen:
- Copy and send them a link via email or chat. If you use Gmail, install our browser extension to enable starting a screen share and inserting the share link directly into your Gmail message.
- Tell them to go to http://www.screenleap.com and enter the 9-digit share code
- Click the "More sharing options", then enter the viewer's email address and we'll send him/her the link for you.
- Click the "More sharing options", then enter the viewer's mobile number and we'll text him/her the link for you.
- As soon as your friends or colleagues click the Screenleap link or enter the share code on the Screenleap home page, they will see your computer screen in their web browser.
How do I stop sharing my screen?
You can stop sharing your screen by clicking on the "Stop" button on the Screenleap console. You will be asked to confirm that you want to end your screen share. If you say yes, your screen share will end and your screen will no longer be viewable by others.
You can also stop sharing your screen by simply closing the browser window or tab that you used to start the Screenleap screen share.
How do I view a screen share?
If someone sent you a link, just click on it! It will open a web page that mirrors the shared screen. If you were
given a 9-digit code, just go to http://www.screenleap.com, enter it into
the share code box, and click the "View screen share" link.
How can I share only a particular application or portion of my screen?
Select the "Share inside rectangle" option on the presenter console to pop up a green rectangle. Your viewers will only see what's inside the rectangle. Resize the rectangle by dragging the left, bottom, or right side of the rectangle. Move the rectangle by dragging the top side.
To share a particular application, click on the application to bring it to the front. Then click on the rectangle to bring it to the front and resize it so that it encompasses the visible area of the application. The rectangle feature is not available on some platforms due to limitations in the graphics support (such as some versions of Linux).
How do I switch the presenter?
If you start a screen share after signing in, you will have the ability to view one of your viewers' screens. Your viewer will see a "Become presenter" button at the top of their screen. When your viewer clicks the button, you will be prompted for permission for the viewer to become the presenter. If you click "Yes", you and all the other viewers on the screen share will be able to see his/her screen. Presenter swap is disabled if you are recording your screen share. You can also disable the presenter swap option from your preferences.
How do I give control to a viewer?
Currently, viewers of your screen cannot take control of your computer.
How many people can I share my screen with at one time?
The Free Account allows you to have up to 8 viewers on each screen share. You can have up to 30 viewers with a Basic Account and the Pro and Premium Accounts allows you to have up to 150 viewers. If you need more concurrent viewers than that on your screen share, you can sign up for a Max Account that allows you to have up to 10,000 viewers on a single screen share. Please contact us if you need to support more than 10,000 viewers on a single screen share.
What is a Screenleap handle? Why do I want one? How do I get it?
A Screenleap handle allows you to broadcast your screen and then allow people to view it by simply going to your handle page (i.e., http://www.screenleap.com/johndoe). You will be able to claim your Screenleap handle when you sign up for an account.
What is broadcasting?
You can only "broadcast" your screen if you have a valid handle. When you broadcast, any viewer can join your screen share by appending your handle to the Screenleap address. For example, if your handle is "steve", you can invite people to view your broadcasted screen by sending them to http://www.screenleap.com/steve.
Can I broadcast with my personal handle, but still control who gets to see my screen?
With a Pro Account, you have two options for restricting access to your broadcasts:
- Before you start a broadcast, you can optionally enter an access code of up to 10 characters. When a viewer goes to your personal broadcast page, they will be required to enter the access code before being allowed to view your screen.
- If the broadcast is already in progress and you have not locked the broadcast with an access code, you will see a "Lock broadcast" button in the sharing instructions. Click this button to prevent any additional viewers from joining. To open it back up again, just click the "Unlock broadcast" button.
What are the benefits of creating a Screenleap account?
While we do not require that you create a account before you can start screen sharing, there are a lot of benefits to be gained from creating an account.
- Users with accounts can claim a permanent handle, which enables them to use a permanent URL to invite viewers to their screen broadcasts.
- Sharing without an account is limited to 40 minutes daily with up to 2 viewers. Create an account to share with more people. If you need more time, you can sign up for one of our paid plans.
- When you do a private share from a Screenleap account, your viewers will see a "Become presenter" button which they can click to share their screen with you. This can be an easy way for you to invite someone else to share a screen with you.
I want to integrate Screenleap into my product? Do you have an API?
Yes! Please see our API Overview for more information.
Is there a way I can initiate screen shares from Gmail?
Yes, our browser extension allows you to start screen shares directly from Gmail. Install the Screenleap extension to be able to start a screen share without leaving Gmail.
Does Screenleap work with multiple monitors?
Yes, Screenleap works with computers with multiple monitors. We only support sharing one screen at a time. The monitor that is share depends on the sharing mode: in full screen mode, the screen that contains the Screenleap console will be shared. In rectangle mode, the portion of the screen inside the green rectangle will be shared.
Can I record my screen shares?
Yes, recording is supported for users on our Premium and Max Accounts. To enable recording, click on the "Recordings" link after signing in and then on the "Enable recording" button.
To record audio, enable audio conferencing and call into the conference line. Audio recording will start when the screen share starts and stop when the screen share ends. If you call the conference line before the screen share starts, the audio will not be recorded until the screen share starts.
When recording is enabled, your screen is automatically recorded when you are sharing your screen. Once you end your screen share, the recording will be converted into a MP4 video file that you can view and download from the "Recordings" page. Each recording is limited to three hours in duration. The Premium and Max Accounts come with 50GB of storage space for your recordings.
If you cancel your Premium or Max Plan subscription, your recordings will be immediately deleted so please download any recordings you want to keep before cancelling.
How much does Screenleap cost?
You can share your screen using Screenleap for up to 40 minutes per day without signing in. If you create a Free Account and sign in, you can share your screen for up to 40 minutes per day. If you need more time or access to additional features, you can sign up for a Basic Account for up to eight hours of screen sharing per day, or sign up for Pro account for unlimited screen sharing.
Do you offer a discount for education or non-profits?
We offer a custom, discounted education solution that is designed specifically for teachers who want to share their screen with their students and recording it for later viewing. Please take a look at our Screenleap for Education solution to learn more.
We don't currently offer discounted prices for non-profits.
How many screen shares can I run at the same time with a paid account?
You can only run one screen share at a time with each paid account. If you try to start up a second screen share while another screen share is already running, the screen share that got started first will automatically be stopped. If you need to support running multiple screen shares at the same time, please sign up for a company account.
How can I get a copy of my subscription payment receipt?
You can view your previous payment receipts by going to the "Account" drop-down menu at the top-right hand corner of the page after logging in and selecting "Subscription". Click on the link titled "Payment History" and then click on the "view" link for the receipt you want to view.
Does Screenleap support audio or voice conferencing?
Yes, audio conferencing is available on all paid plans. To use it, simply click on the "Show instructions" link under "Audio Preferences" when running a screen share. In addition to audio conferencing, we also support one-way broadcast audio where we share the audio captured through your computer's microphone with your viewers.
We do not currently support video conferencing where you can see the people you are speaking with.
What is audio conferencing?
Audio conferencing is a paid feature that allows you to speak with all the participants of your screen share. Audio conferencing requires that all participants call into a conference number after the screen share has started.
How do I start an audio conference call?
You can start an audio conference call by first starting a screen share. You will see a section named "Audio conferencing" below the sharing instructions. Click the "Show instructions" to see the dial-in number and access code that you will need to enter when prompted. We currently have dial-in numbers for 55 countries.
How much does audio conferencing cost?
Audio conferencing is included in all paid plans. Please beware that the dial-in numbers are not toll-free so there may be long distance charges associated with calling the number that will be reflected in your telephone bill.
Can I reserve my own conference ID?
Yes, if you plan to use the audio conferencing feature regularly, you can make getting started easier by reserving a conference ID that you will use for every call. We will show you and your viewers the same conference ID for every screen share.
How secure are my audio conference calls?
We assign you a 7-digit conference ID that can be used to connect to a conference bridge. Conference calls are currently not secured by any additional PIN codes. While it may be possible for someone to join your conference call if they guess your conference ID, we don't think it is very likely in practice since there are more than 10 million possible conference ID combinations. As a secondary safeguard, we will also notify you when someone joins your call so you will be able to verify the new caller if you were not expecting anyone else to join.
Are there additional features that I can use during as the call host?
Yes, if you are the presenter, you can gain access additional commands during your call by entering * 3 followed by your
host PIN. A host PIN is a 4-digit code that you can select when editing your preference. The available commands are:
|Lock/unlock - prevent additional callers from entering call|
|Listen-only mode - only the call host can be heard|
|Conversation mode - all callers can be heard|
|Mute/unmute your line|
|Play entry chime on caller arrival|
|Announce name on caller arrival|
|No notification on caller arrival|
|Play exit chime on caller exit|
|Announce name on caller exit|
|No notification on caller exit|
Do you have a list of conference call dial-in numbers for different countries?
Yes, please see the list of conference call numbers.
What is broadcast audio? How does it work?
Broadcast audio allows you to share audio captured from your computer's microphone with your viewers. You no longer need to call a conference number in order to share your audio like you do with audio conferencing. The limitation, however, is that broadcast audio is one-way and you will not be able to hear your viewers. Like our screen sharing functionality, your viewers do not need to install any software in order to hear your broadcasted audio: all they need is a web browser.
Broadcast audio is available on all paid plans. You can broadcast your audio from all supported sharing devices, including Windows, Macs, Chromebooks, iOS, and Android devices. To enable broadcast audio, go to your "Settings & Preferences" and select the broadcast audio option in the "Audio preferences" section.
Please note that broadcast audio is one way and that you will not be able to hear your viewers.
Viewer Mouse/Control Sharing (Collaborative Screen Sharing)
What is the viewer mouse feature? How does it work?
The viewer mouse feature allows you to collaborate with your viewers by allowing you to give permission to one of your viewers to point things out on your screen. Once you have viewers viewing your screen share, you can give one of your viewers permission to show his/her mouse cursor on your screen by clicking on the mouse icon on your console, selecting the viewer to show the viewer mouse for, and then selecting "Show viewer's mouse cursor". You will be able to see the viewer's mouse cursor on your screen so that your viewer will be able to point things out.
You can stop seeing your viewer's mouse cursor by clicking on the mouse icon again and selecting "Disable viewer mouse".
What are the limitations for the viewer mouse functionality in the Chrome extension?
Because of limitations in the Chrome browser, viewer mouse is only available when you are sharing your browser window. It is not available if you select to share your entire screen.
What is control sharing/collaborative screen sharing? How does it work?
Collaborative screen sharing is a feature that allows you to share control of your computer with one of your viewers for collaboration purposes. You and your viewer will each have your own mouse cursor to control. You will both be able to interact with your screen using your respective mouse and keyboard at the same time. Collaborative screen sharing is available on all paid plans.I've shared control with my viewer. Why can't he/she interact with my computer?
You will need to make sure that the viewer page is the active window and the active tab. If another window is in front of the viewer window, then your viewer's actions will not be passed to your computer.
How can I capture contact information from my viewers so I can follow up with them?
The viewer registration features allow you to capture information about your viewers before they are allowed to view your screen share. This is especially useful if you are using Screenleap for marketing and sales webinars as you will be able to export the viewer registration information to your CRM after your presentation so you can follow up with them.
To enable viewer registration, sign into the Screenleap website select, select Settings & Preferences from the Account menu, and then select what information your viewers are require to provide. You can require your viewers to provide the following information:
- Name and email address
- Name and phone number
- Name, email address, and phone number
- Name, company, and email address
- Name, company, and phone number
- Name, company, email address, and phone number
When your viewers go to view your screen share, they will be prompted for the information you specified as being required before they are allowed to view your screen.
How do I uninstall the Screenleap Chrome extension?
To uninstall the Screenleap extension:
- Type "chrome://extensions" into your address bar and press "Enter".
- Click the trash icon for the "Screenleap" extension.
Screenleap for iOS
How do I perform the initial setup to allow for sharing my iOS screen?
In order to share your apps when sharing your screen using the Screenleap for iOS app, you need to perform some initial setup. See the instructions for enabling sharing of your apps on your iPhone.
How do I share my screen in landscape mode when using the Screenleap for iOS app?
In order to share your screen in landscape mode when using the Screenleap for iOS app, you will need to rotate your device to the orientation you want while you're in the Screenleap app. After you have rotated your device to your preferred orientation, you can then leave the Screenleap app and navigate to your app to share it. You will not be able to change the orientation of the screen share once you leave the Screenleap app. To change the orientation, navigate back to the Screenleap app and change the orientation and then return to the app you are sharing.
Screenleap for Mac
My Mac has MacOS Catalina (10.15) or later. When I share my screen, my viewers only sees the background of my desktop. How can I allow them to see my windows as well?
MacOS Catalina (10.15) requires that you grant permission for the Screen Recording to the Screenleap for Mac app.
To grant permission:
- Open your System Preferences.
- Click on Security & Privacy.
- Click on Screen Recording.
- Check the box for the Screenleap app. You will see a dialog saying that you need to restart the Screenleap app, but you do not need to. You can just continue your screen share.
How do I grant permission for sharing control to my Screenleap for Mac app?
In order to allow your viewers to share control of your screen, you will need to grant permission for the Accessibility permission to the Screenleap for Mac app.
To grant permission:
- Open your System Preferences.
- Click on Security & Privacy.
- Click on Accessibility.
- Check the box for the Screenleap app (you may need to click the lock icon and enter your password to allow for checking the boxes).
Why can't I grant the Accessibility permission to the Screenleap for Mac app?
If you open your System Preferences and can't select the Screenleap app from the Accessibility option on the Security & Privacy page, it is likely that your privacy database has been corrupted.
- You can fix it by opening the Terminal app and running:
rm -f /Library/Application Support/com.apple.TCC/TCC.db
- Click the Screenleap icon on your menu bar and select Quit Screenleap.
- Start a screen share from the Screenleap website and click Open Screenleap.app when the Open Screenleap.app prompt appears.
Are my screen shares secure? What's to prevent unauthorized users from viewing my screen?
Any user with access to the share code for your screen share will be able to view your screen. We use a long share code to make it difficult for unauthorized users to stumble upon your screen share but it will not completely remove the chance of it occurring. If you need additional security, we offer the ability to require an access code to view a broadcast for users with a Pro Account.
Is my screen data going through Screenleap's servers? Is it stored there?
Screenleap sends your screen data to our servers so that it can be sent to your viewers. All data is kept in memory and is never stored to disk. Your data is cleared from memory when you end your screen share or after two minutes if your screen share is not ended cleanly.
I ended my screen share by closing my browser instead of using the "Stop" button. When I click on the screen share link I see the last image of my screen. How can I remove it?
If you end your screen share by closing your browser, we will retain the last screen image for about two minutes. This is because we do not get notified that your screen share has ended if you just close your browser so we are unable to determine if you intended to end your screen share or if it's just a temporary connection problem. To prevent accidentally clearing out images for temporary connection problems (and incorrectly showing your viewers that the screen share has ended), we do not remove the last screen image from our servers until after two minutes. For this reason, it is recommended that you end your screen share by pressing the "Stop" button.
Are the connections for my screen shares encrypted?
All screen shares are secured using strong TLS 1.2 256-bit AES end-to-end encryption. TLS is the same encryption technology that banks use to protect your sensitive information.
If I host or view a Screenleap screen share, can the presenter or viewer control, install, or download things from my computer?
No, screen shares are one-way broadcasts. Viewers cannot control, install, or download anything on the host's computer and vice versa: hosts cannot control, install, or download anything on the viewer's computer.
Are my screen shares ever shared with a third party?
No, your screen share will never be shared with a third party.
How does Screenleap compare with other screen sharing tools like WebEx for performance?
Screenleap is designed as the easiest way to share your screen. In order to achieve this, we had to push a lot of processing to our servers so it won't perform as well for instances where you need a video-like experience. There may be other screen sharing tools that work better for these instances. Screenleap works really well however for things like sharing a PowerPoint presentation, getting feedback on a design, reviewing an Excel spreadsheet, or giving a sales demo of a web application.
Can I use Screenleap to share a video in real-time?
Screenleap is designed as the easiest way to share your screen. Our technology choices are focused on this aspect. You may be able to get somewhat acceptable video performance if you and your viewers have a fast computer and a fast connection but it's not what it's designed for.
How can I improve the performance of my screen share?
Screen sharing is bandwidth-intensive so if you are on an Internet connection with better bandwidth, you will experience better performance. In addition, we recommend that you use a land-line or cell phone to speak with your viewers when possible during a screen share because audio is also bandwidth-intensive and running both through your Internet connection at the same time will reduce quality and performance of both.
Another thing you can do to improve performance is to use the "Share inside rectangle" feature when sharing to reduce the portion the screen that is being shared to the region that you are interested in. The smaller the area being shared, the faster the performance and the lower the latency.
Finally, you can improve the performance of your screen share by making sure that your viewers are using the latest version of their browsers. Screenleap's viewer is built using HTML5 and we take advantage of many advanced HTML5 features to improve performance. By keeping your browsers updated to the latest version, you can ensure that you are getting the best performance.
Is there a maximum screen size that's allowed to be shared with Screenleap?
Yes, in order to ensure adequate performance, we currently restrict the maximum size of the screen being shared to 2560 pixels wide x 1600 pixels high. If your screen is larger than this, you will not be able to use the full-screen mode and the screen share will automatically default to the rectangle mode. The rectangle will also be restricted to a maximum size of 2560 x 1600.
We recommend making the shared area as small as possible to ensure the best possible performance.
Which browser has the best performance?
Screenleap currently works best with Google Chrome, followed by Firefox, Edge, Safari, and Internet Explorer. If you are not already on Google Chrome, we recommend installing Google Chrome to achieve the best screen sharing performance.
I'm a non-U.S. user, why is Screenleap so slow?
Currently, our servers are hosted in only the United States, Europe, and East Asia. If you are sharing with users in India, Australasia, South America, or Africa, you may experience increased latency. We will be adding servers to those regions in the future as well as adding support for direct peer-to-peer connections so that screen shares between presenters and viewers in those countries are faster.
The Screenleap for Windows app is crashing for me. How can I help track down the problem?
If the Screenleap for Windows is crashing for you, you can help us to track down the source of the crash by enabling the generation of crash report whenever a crash occurs. To enable the crash report, download and install the Screenleap crash report installer and run it as an administrator on your computer.
Perform the following to run the installer as an administrator:
- Click on the Windows "Start" button and type "command" in the "Search programs and files" box.
- Locate the "Command Prompt" shortcut and right-click on it.
- Click on "Run as administrator".
- Navigate to the directory where you saved the downloaded crash report installer file and run it.
If the Screenleap for Windows crashes again, a crash dump report will be generated in the C:\Users\[your username]\AppData\Local\CrashDumps directory. The crash dump file will be named screenleap.exe.dmp. Please email us the file so we can track down the cause of the crash.
I'm trying to share my screen from a VirtualBox instance but my viewers only see a black screen. How do I fix it?
Please try disabling 3D acceleration for your VirtualBox instance.
I accidentally checked "Remember my choice" and then clicked "Do Nothing" when asked to launch the application. How can I get the dialog to appear again?
We require the custom protocol handler to be enabled for screen sharing to work. If you accidentally checked "Remember my choice" and clicked "Do Nothing", you will need to reset the setting in order to share your screen:
- Quit Chrome completely. If there's any Chrome instances running, the reset steps will not work.
- Delete the "Local State" file in ~/.config/google-chrome.
- Launch Chrome and try sharing your screen again.
When I am sharing my screen, the viewers are seeing the screen zoom in and out constantly. How do I fix the issue?
Your issue is likely because you are on a computer with multiple screens, the monitors have different DPIs, and you are sharing your screen using the "Select window" option. Depending upon the placement of the window that you are sharing, the operating system can switch back between the two screens rapidly, which causes the shared window to appear for the viewer as a zooming in-and-out of the screen.
The fix is to use either the "Share entire screen" or "Share inside rectangle" options.