I never known what is this issue and what keyword to use for searching. It's my appreciation for your help. This tutorial works for me and this issue is resolved.
However, I still did not know how it happened because it just occurred recently. I used a same laptop with Tor browser in many months and did not get such issue. It is why I wrote in OP that this issue can be mistakenly activity from my side. Do you think so?
Without knowing your setup we can only really speculate. Though, it might be due to differences in versions, Tor Browser has regular updates, and if you've updated recently the letterboxing might work or behave a little differently on your device from the last update.
Alternatively, you probably just haven't noticed or it might behave differently depending on the size you resize the Tor Browser window since reducing or increasing it might impact how noticeable it is.