Unable to mark a quote box

[Kastor Krieg]

As I was adding this post #1, I've attempted to put the excessively large quote into a spoiler block as I was concerned about the size (I didn't know about the Expand feature of the new forum). There has occured a weird behaviour however when I've attempted to mark the whole quote block with both mouse and keyboard and it failed to hold, "slipped off".

REPRO:
1. Create a Quote Reply to another Frater's post.
2. Write response text under the automatically created quote block.
3. Format the response text with the Bold, Italic and Color>More Colors> #ff0000 primary red.
4. Go to this post #2 and quote it fully with the Quote button.
5. When moved to the Reply box below, with the full quote visible already as a Quote block, press Ctrl+A to mark all
6. Press Ctrl+X to Cut the entirety of Reply box contents, with all the formatting.
7. Go to the initial post #1 and press Ctrl+V to Paste the entire Quote block of post #2, with all the formatting.
8a. Use your mouse and left click, dragging from empty lines above the post #2 Quote block, all the way to the empty lines below.
  This is in an attempt to mark the whole Quote block and click the Spoiler button.

OR

8b. Click on the empty line above the Quote block you want to mark. Then press Ctrl+Shift+PageDown in an attempt to mark the same Quote block.
9. The blue mark indication appears and as soon as the mouse button (8a) or keypress combination (8b) is lifted, it disapppears. You are thus unable to put that Quote inside a Spoiler or another Quote.

EXPECTED RESULT:

9. The entire Quote block is marked blue and remains marked after mouse button (8a) or keypress combination (8b) is lifted. This permits the user to nest that block inside another Quote or a Spoiler block.


SIDE NOTES:
The quoted post (#2) contains a direct paste of the book quote from a Reddit post, which has resulted in a different type of vertical quote marks before each non-break line. This may have affected the behavior of the text mark functionality. The issue as persisted both during initial writing of post #1, as well as during subsequent Edits, in current and new tabs of the browser, before and after Ctrl+R forced full refresh. 

The browser is the most current basic (non-GX) Opera browser [ 91.0.4516.20 ], running on Chromium. An image of the weird in-line Reddit quote formatting below.

[sibomots]

Thanks for the details.

I'll try to troubleshoot this.