While English Tiger's approach works and is pretty fast, I discovered an even easier/quicker way...
After you right click in the main display window to bring up the visualizations/trackinfo context menu, there will be an "Enable Scrollbars" item 2nd from the bottom. It may or may not have a checkmark next to it. Whichever it is, just select that and it will refresh the current trackinfo template you are using. This saves you from having to navigate to the context+sub-menu twice (once to select a different template, and then again to re-select the one you are developing).
FYI: whether Enable Scrollbars is active (checked) or not, it does not seem to affect/change the trackinfo templates behavior in any way that I can find. If you have scrollbars in your template and you uncheck it, it will still have scrollbars. If you don't have scrollbars in your template, activating this setting does not magically add them. So it's stated function is inactive, but it is a super convenient way to "refresh" the display window! Perhaps JRiver should change the menu item name from Enable Scrollbars to Refresh Display?
Final note... the Enable Scrollbars menu item only shows if you are using a trackinfo template (any template). If you are showing the Cover Art (with or without lyrics), MC's built-in visualization (3D or standard), the item will not be listed in the context menu.