Steps how to reproduce with Installation of Version: 7.6.4.1 (X86_64) 
Build ID: e19e193f88cd6c0525a17fb7a176ed8e6a3e2aa1
CPU threads: 12; OS: Windows 10.0 Build 19045; UI render: Skia/Raster; VCL: win
Locale: Locale: de-DE (de_DE); UI: de-DE; Theme: Automatic (Colibre)
Calc: CL threaded – Normal User Profile
PC: AMD Ryzen 5 3600 6-Core Processor 3.60 GHz, 16,0 GB RAM

0. Download and unzip TestKit from "Bug 160485 - FILESAVE: Some Cells Text missing in EXPORT.png with highresolution."
1. Open SampldDocument.ods
2. Click cell CX3 and with select range  CX3 - A1
3. Release Mouse Button
   » Area keeps highlighted / selected
4. Menu ˋFile → Export → Format = WEBP → (Keep Auto-Filename-Extension and
   "Selection" checked → Exportˊ
   » Export Dialog appears
5. ˋRadio button "Modify Dimensions" → in Width add a "00" (2 times zero) left from comma → <TAB> for reaching Height fieldˊ
   » Height value also becomes more or less ten times as much, similar to 
     width.
6. ˋCheck "Lossless" if not already done → [ok]ˊ
   » Expected: WEBP should be be created
7. Open PNG in IrfanVIEW
   Actual: Endless exporting. After 50 minutes still not done 😥. 
           I terminated LibO.

Additional info: 
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a) before this test I tried with several files open; LibO crashed.
b) I was not able to close the source calc document via WIN Task Manger Which successfully r Task list, but document kept open) or in any other way, had to reboot the PC