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A utility that remotely displays the viewport size of a browser window and the mouse position within the viewport.
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Web Page Size/Dimension/Mouse Position Viewer

A utility that remotely displays the viewport size of a browser window and the mouse position within the viewport.


This tool will allow you to have a live view of the width and height of a separate browser window. It utilizes the browser's local storage and storage events to communicate changes in the view port size to a separate instance of the browser.


I was working on some responsive web pages adding some media breaks in order to fine tune the responsiveness. And I needed to see the width of the of the viewing area to get the numbers I needed.

My method was to resize the window until the layout of the elements became undesirable, stop resizing and note the current width of the view area.

I use Chrome for development so I searched the extensions for one that would provide live reporting of at least the width. But nothing worked as I needed. So I tried to think of ways to use JavaScript to convey data between tabs or browser instances. I knew that local storage is shared between pages from the same domain. But polling for data updates wasn't something I wanted to use here.

After some web searches and some time I found out about local storage events. And what you see here is the result of what I learned.

Running the Viewer

Requirements :

If using Chrome, IE, or Edge :

The files must reside on the same server. And accessing them directly from where they're stored on your PC will not work. For example file:///C:/path/to/index.html will not work. But http://localhost/path/to/index.html will work.

If using FireFox :

You can either host the files or load the page via file:///C:/path/to/index.html. Either method will work.


1) Create a folder called temp in your server's public HTML folder.

2) Copy both of HTML files and the assets/js/ path and its file(s) to temp.

3) Open the index.html file in your browser and follow the instructions found there.


The viewer will display the following -

  • The name of the page. For example if your getting data from index.html then index will be seen at the top of the page.
  • The width and the height of the page when it was first loaded.
  • The width and the height of the page, the values will be updated as it's being resized.
  • The current mouse position over the viewed page.

Viewer Example #1

Browser Behavior

Except for FireFox all of the other browsers I used behave as expected in regards to reporting the viewport size and mouse position. The only discrepancy I saw was the mouse position. When Firefox was used the mouse position would stop reporting below a fixed "line" within the browser viewport. Where other browsers would report the position across the entire viewport.

The cause of the difference in behavior is determined by the particular browser's user agent stylesheet. This is the CSS styling that is used when the page has no CSS of its own. The following images illustrate the differences, the highlighted areas represent the document body.

Chrome :

Viewer Example #2

FireFox :

Viewer Example #3

Real-World Usage

The following shows the minimum changes that need to be made to any page where you need to view the size and mouse position -

    <meta charset="UTF-8">
<body onresize="showsize()">
    <div id="viewsize-link">
    <script src=""></script>
    <script src="assets/js/viewsize.js"></script>

Future Development

This was a fun and useful project! And it got me to thinking about how else I could use local storage events. So after a few minutes I came up with what follows...

Appearance Improvements

The look of the viewer is a bit rough, but it is just a tool after all. However, when I've got the time I'll work on the appearance and pretty it up a bit.

Semi-Remote Logger

I might try replacing console.log() calls with something that will send the output to local storage. And then a remote viewer would display the output.

I will likely need to queue up messages for situations where multiple calls to console.log() are made in quick succession. And I'll use session storage instead of local to be sure the queue is deleted when the browser windows are closed.

Miscellaneous Design Details

  • The viewer will set a flag in storage that indicates that it's present and ready. The remote-enabled page will check the flag and if not present then nothing will be saved to storage.
  • TBD

Semi-Remote CLI

I can't really say how useful or practical it would actually be to have a CLI for a web page. But it might have its uses. For example -

  • For debugging & testing -
    • Alter run-time settings
    • Pause/resume execution
    • A learning tool

Recommended Reading

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