scrolling (using scroll bar) is much slower than old versions

[gaby-de-wilde]

Expected behavior

When grabbing the scroll bar and moving it up or down the chat text should instantly scroll to the position corresponding with the scroll bar position.

Actual behavior

It takes 7+ seconds to move to the top from the bottom.

Steps to reproduce the issue

1. join a busy channel
2. wait for the chat to fill up (so that old lines are removed from the top)
3. grab the scroll bar with the mouse
4. move mouse up and down.

System information

KVIrc 4.9.2 'Aria'

Runtime Info:
System name: Linux 4.4.0-38-generic
System version: #57-Ubuntu SMP Tue Sep 6 15:42:33 UTC 2016
Architecture: x86_64
Qt version: 5.5.1
Qt theme: fusion

Build Info:
Build date: 2016-09-16 22:11:36 UTC
Sources date: 20160102
Revision number: git-6984-gf8f50bd
System name: Linux-3.13.0-95-generic
CPU name: x86_64
Build command: /usr/bin/cmake
Build flags: 
   MANDIR=share/man
   CMAKE_INSTALL_PREFIX=/usr
   LIB_SUFFIX=/x86_64-linux-gnu
   Threads=POSIX
Compiler name: /usr/bin/x86_64-linux-gnu-g++
Compiler flags: -g -O2 -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 
Qt version: 5.5.1
Features: IRC, Debug, IPv6, Crypt, SSL, IfAddr, IPC, Transparency, Phonon, Webkit, DCCVoice, Perl, Python, Enchant, Qt5, KVS

OS information

Ubuntu 16.04 LTS

[un1versal]

This would depend on how big the buffer is, normally without modifying default buffer, grabbing the scrollbar and scrolling all up/down takes as long as it ever took here.

I cant duplicate this slowness you report here on same OS and my buffer is ~~5K~~ 9999K lines.

[un1versal]

Still cant duplicate this slowness reported here with buffer set to max 9999K lines on different servers both WIndows and Linux Ubuntu builds.

[IceN9ne]

I can confirm. It has been an issue for a while now and I've seen it on Ubuntu 15.10 and 16.04. If you grab the scrollbar and drag it up and down for more than a couple of seconds (like if you're searching for something), the scrollbar lags significantly and can take a up to a couple of seconds to catch up to your mouse.

Note: This is on a high performance computer as well, so not resource limited.

[un1versal]

On a VM with 2 CPU and 2GB ram

cant see this and I been grabbing scrollbar up and down for more than 2 seconds, never lagged so far. Doesnt mean issue isnt there just that I cant dupe it.

[IceN9ne]

I just did some more testing, and it only happens when ~~KVIrc is maximized~~ the IrcView has a large drawing area.

Edit: The larger the screen size, the more obvious it is.

[un1versal]

kvirc taking 1920x1080, no split view, not maximized but made ircview as big as possible, still vm with 2 cpu and 2gb ram,

• joined a extremely noisy channel being fed some 20 lines per second (maybe more)
• waited for buffer to fill full 9999k lines (maximum)
• grabbed scrollbar nib and scrolled up and down at variable speeds slow to faster to slower again

no slowness, no lag and definitely no 7 seconds to go fully all the way up (best all way down has more lines still coming in anyway) or reverse.

Any bigger screen area is possible here for tests, but I doubt most users will have 4K screens.

The only difference here is I compile my own Linux versions, but windows version is same as offered on official site,