Small Thumbnails on second 4K monitor
ProSelect's Mirror Display Area mode is a great feature for showing your images on a second screen to your clients without them seeing all the other tools, order forms etc that you are using.
However, if you have selected to show thumbnails (Thumbnail View) on the second screen and there is a large difference in the resolution between your main and second screens, then the overall size of the thumbnail image will be limited to their size on the smaller of the two screens.
For example, if you have a laptop with a 1920x1080 sized screen and are connecting to a 4K UHD monitor with a resolution of 3840 x 2160 then your thumbnails will appear centered at around half size on the 4K monitor. This is because, in order to keep the display layout the same on both screens, thumbnails are copied pixel-by-pixel between the two screens.
The solution is to change the display settings of the larger screen to be as close as possible to the smaller screen. This is normally done on your Display Settings
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KnowledgeBase Article: Small Thumbnails on second 4K monitor