1. Create a new text document 2. In page style, set area to black color 3. Optional (for better visibility): set page size to 5x5 cm 4. Export the document to PNG (with default settings, like 96 PPI, any compression, interlaced, save transparency). The resulting PNG has a grey border at bottom and right.
Note that the page style uses "Background cover margins" option [1]. [1] https://git.libreoffice.org/core/commit/56d8007a197b095b09423c691a51515567648e80 by Michael Stahl
Confirmed with: Version: 7.4.0.2 / LibreOffice Community Build ID: 1512ce97d7ed39dce3121f7e15651fd8895f950e CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3 Locale: en-AU (en_AU.UTF-8); UI: en-US Calc: threaded Looks like it's a border of one pixel width of grey and one pixel width (outermost) of full transparency (see attachment).
The transparent border is in 7.4.0.2 but not in 7.2.7.2. Only the grey border in that one. Version: 7.2.7.2 / LibreOffice Community Build ID: 8d71d29d553c0f7dcbfa38fbfda25ee34cce99a2 CPU threads: 8; OS: Linux 5.15; UI render: default; VCL: gtk3 Locale: en-AU (en_AU.UTF-8); UI: en-US Calc: threaded
Created attachment 181732 [details] zoom on overlapped border after export by 7.4.0.2 PNG export is overlapped on other picture in GIMP. 1 pixel-wide grey band and 1 pixel-wide transparent border are visible.