Patrick, the issue you are facing is not due to OS or browser, it more an about your browser window's aspect ratio. When setting a map's location, there are two approaches:
1. Set the map's center point and scale. In this case the scale will remain constant but the extent of the map vary greatly across different aspect ratios. For example, if you want to show the same scale on a map of the United Sates at different aspect ratios, you see this difference in extent:
Widescreen Desktop
Widescreen Portrait (smartphone)
The east and west coast would get cropped
2. The second method is to maintain the extent but change the scale to fit that same extent in a different aspect ratios. So to fit the United States in a tall screen, the map needs to zoom out:
In general, most users find preserving extent (method 2) more important than preserving scale (method 1) which is why we chose this method in our templates. In addition to aspect ratio, when you are using raster tiles in your map, the map can only be displayed at the exact scales your tiles have been rendered at (usually 19 levels) as apposed to showing an intermediate scale for best fit.
When you save the map's position in the cascade, we save the coordinates from the four corners of the map size you see in the builder. When the app sets the extent of the map to the extent your saved, it will finds the scale with the extent that has the closest match to the extent you saved. In some cases this does cause minor "cropping" around the map but limits the amount of extra buffer space on the map that's not important.
Here's a workflow that should limit the buffer space around the map but ensure the features in the map shows at all aspect ratios.
1. Resize your browser window so the visible map area's aspect ratio is closest to the aspect ratio of the features you want to show in the map.
2. Zoom and center your map on the features you wish to show. There should be as little buffer space as possible around all four sides of the features in the map.
3. Use the "Zoom Out" button to zoom out the map one level (this will maintain the center of the map).
4. Save this extent.
As Owen mentioned, we will look to provide a better experience in the future and possibly provide more options for authors to choose how the extent of the map is set. Would it be better for the map to zoom out farther to make sure the full saved extent is always shown rather than finding scale with the closest match even if it crops some of the edges?