y-scrollbar, wrong initial position

I cannot recreate here. Can you start the video before you launch pcbnew?

Application: kicad
Version: (5.1.0-rc1-111-g203486638b), release build
Libraries:
    wxWidgets 3.0.4
Platform: Linux 4.19.0-2-amd64 x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.67.0
    OpenCASCADE Community Edition: 6.9.1
    Compiler: GCC 8.2.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=ON
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=OFF
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=OFF
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=OFF
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

That's actually the status after just opening pcbnew, without doing any pan or zoom action. After doing such an action the scroll-bar position is correct.

Does it happen in any other applications?

I can't reproduce

Application: kicad
Version: 5.1.0-rc2-unknown-2768b3d~91~ubuntu18.10.1, release build
Libraries:
    wxWidgets 3.0.4
    libcurl/7.61.0 OpenSSL/1.1.1 zlib/1.2.11 libidn2/2.0.5 libpsl/0.20.2 (+libidn2/2.0.4) nghttp2/1.32.1 librtmp/2.3
Platform: Linux 4.18.0-15-generic x86_64, 64 bit, Little endian, wxGTK
Build Info:
    wxWidgets: 3.0.4 (wchar_t,wx containers,compatible with 2.8) GTK+ 3.24
    Boost: 1.67.0
    OpenCASCADE Community Edition: 6.9.1
    Curl: 7.61.0
    Compiler: GCC 8.2.0 with C++ ABI 1013

Build settings:
    USE_WX_GRAPHICS_CONTEXT=OFF
    USE_WX_OVERLAY=ON
    KICAD_SCRIPTING=ON
    KICAD_SCRIPTING_MODULES=ON
    KICAD_SCRIPTING_PYTHON3=ON
    KICAD_SCRIPTING_WXPYTHON=ON
    KICAD_SCRIPTING_WXPYTHON_PHOENIX=ON
    KICAD_SCRIPTING_ACTION_MENU=ON
    BUILD_GITHUB_PLUGIN=ON
    KICAD_USE_OCE=ON
    KICAD_USE_OCC=OFF
    KICAD_SPICE=ON

Does this still happen?

Does this happen in different window managers?

Ok, now I've hit this a few times. It appears to relate to how we calculate the position on various platforms. Since the scroll thumb for GTK3 is variable size, we need to adjust how much size is available, not the size of the scrollbar.

Fixed in revision 85df994eda4caae28049e92d9d48e7d45d160605
https://git.launchpad.net/kicad/patch/?id=85df994eda4caae28049e92d9d48e7d45d160605