scrollbar bug with alternate buffer

[jerch]

Repro:

• run demo
• open vim, .e.g. call vim package.json
• scroll with the mouse several times up and down

--> At some point a scrollbar appears and messes further scrolling with the mouse totally up.

Expected:
On alternate buffer applications a scrollbar with a scrollbuffer handling should never appear.

Observations:
Seem I can only trigger the faulty behavior once. After exit to the shell and re-running vim I cannot provoke it again.

@Tyriar My first guess was, that something with the dimensions is off in the demo. But I can also repro it in vscode, so it actually might be related to the new scrollbar impl and/or related to the fake-scrolling with arrow keys (edited).

scrollbug2.mp4

[Tyriar]

@jerch can't reproduce this on master or #5411 branch. Do you have anything interesting set in .vimrc? Something I do observe is when vim mouse events are off then the mouse event's default is incorrectly not prevented so it also scrolls the page.

[jerch]

@Tyriar Yes basically that - mouse events are off by default (I rarely use those with vim). What bugs me - why would the scrollbar appear at all for an alternate buffer app? Normally the alternate buffer should not have a scrollbuffer at all, so something is weird here.

[Tyriar]

I mean I didn't see what you saw; no scroll bar at all but scrolling would scroll the web page.

[jerch]

I mean I didn't see what you saw; no scroll bar at all but scrolling would scroll the web page.

Hmm ok, thats weird. I gonna check again on master and test your PR, maybe it fixes things.