I think what you want is, you scroll with mouse, and expect that vim keeps the cursor in original place. E.g. your cursor is at line 5, and you scroll down 5000 lines, you expect your cursor is still at line 5. That is, the cursor is out of the window.
AFAIK, the cursor won't go out of the window. That means, if you keep scrolling down, and the cursor line will be the top line of your current window. and the rnu are gonna re-calculated by the cursor line.
May be you could just explain what do you want to do. the cases in your question could be done by 200dd
or 200Y
but I guess it is not as simple as that.
You may want to find out the ending line by reading/scanning your text lines, and pick the line number (rnu), and do a xxxdd
if this was the case. Here you should use normal line number. e.g. your cursor was at line 5, and you scroll down a lot, find the line you want to delete till from line 5. you could do :5,.d
vim will delete from line 5 to your current line
.
Or you can do 5, 23452d
if you find out the lines between 5 and 23452 need to be removed.
If you can search your ending line by /pattern
search, vim can do :.,/foo/d
this will delete from current line till the next line, which matches foo
.
You can still press V
enter line-wise visual mode, and moving down by vim-motions. when it reaches the point you want to remove/yand press Y or d
You can take a look this Question/answer:
VIM to delete a range of lines into a register
At the end, I suggest you not using mouse in vim.