Background auto-completion means that glossary entries taken out upon glossary pretranslation are used for auto-completion purposes only, and are not shown as term matches "in the source pane," but in the Match Bar.
Suppose you have one or more large glossaries that are used for the general purpose or auto-completion purpose only.
If every match from them is highlighted in the source pane, the result is no more than a color jungle there.
If those term matches are used only for auto-completion without color highlights, you can focus more on ProjectTerms and/or other more relevant glossaries.
You're gonna see a very long Match Bar (showing all pretranslated glossary matches), but the source pane is calm (non-distracting) enough to let you focus on what you really need to focus on.
How about View > Appearance > Colors > Hide glossary match colors in the source segment? BTW, the latest update displays the Japanese auto-completion panel at the top of the screen to avoid overlapping with the system auto-completion.
(Screenshot is not an EN_JA project, sorry)
Thanks for your attention