1. In a Writer document, make sure that Word Completion is enabled in Tools->AutoCorrect->AutoCorrect Options 2. Type some lengthy words (>= 8 characters by default, like "reasonable") to get them collected by Word Completion feature 3. Open Tools->AutoCorrect->AutoCorrect Options on Word Completion tab, see that there are collected words in the list, none of them selected, but the Delete button is enabled. Pressing on that button does not remove anything unless you select a word in the list. The Delete button should be disabled whenever there's no selection.
Confirmed with Version: 7.5.0.0.alpha0+ (x86) / LibreOffice Community Build ID: 550392aeb849b326aa0d5d84a0ec1d28d3d42503 CPU threads: 4; OS: Windows 10.0 Build 19044; UI render: Skia/Raster; VCL: win Locale: ro-RO (ro_RO); UI: en-US Calc: threaded
Code pointer: OfaAutoCompleteTabPage in cui/source/tabpages/autocdlg.cxx; it calls m_xPBEntries->set_sensitive only when the list is empty - the solution would be to implement a selection event handler for the list, and set sensitivity in it.
In my opinion the first entry should be preselected, instead of disabling the button and enabling it after some selection.. At the point you have selected some entry the collected word list, you can't leave without having something selected anyhow (except you delete every collected word) --- Side-note: delete from list doesn't mean it being instantly applied. The word completion keeps suggesting the word deleted, until you quite LibreOffice and restart (see bug 67826)
(In reply to Telesto from comment #3) Or that - would be even easier to implement :)