I was using it for months, but as long as I’m asking, is there a way to make a document save with white spaces hidden and then reopen with them also still hidden? The most important question is what do I have to do to make it usable again?
This view “aspect” makes sense only if Writer can totally eliminate pages, i.e. you don’t impose otherwise a multi-page view.
At bottom right of the status bar, there is the zoom slider: 
. Immediately at its left, there is a group of 3 small icons for single-page, two-pages and book views. In the picture above, book-mode is enabled.
Multi-page view disables View>Hide Whitespace. To make it active again, switch to single-page mode.
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When this is not working, you must switch from View Layout Automatic to Single page.
View Zoom Zoom View Layout Single Page OK