If you're trying to preview a page and the preview window is very short like the screenshot above, you're likely running into a Chromium bug that results in iframes being rendered at 100px in height instead of 100%. This issue is usually seen in Chrome on Windows.
Workaround #1 - Use an alternate browser.
Until this bug is resolved in a future release of Chrome / Edge, please try using an alternate browser such as Firefox.
Workaround #2 - Try adjusting the size of the browser window.
Some users report changing the size of the browser window will cause the 100% height to be respected.
Note - The following former workaround is no longer available in more recent versions of Chrome where LayoutNG is no longer experimental:
If using a different browser is not an option or not preferred, you can also disable the
enable-layout-ng flag in Chrome's settings:
Open a new tab and visit the URL
enable-layout-ng; this should return a single item named "Enable LayoutNG" as seen below:
In the drop down menu to the right of that property, change the selection to Disabled.
Re-launch Chrome (using the button that appears at the bottom right-hand corner of the page).