I just installed (with dnf) Avidemux 2.7.1 (avidemux3_qt5) on Fedora 28 with GNOME 3.28.1 on a ThinkPad X240 which has a display resolution of 1366x768. The minimum height of the Avidemux window exceeds the height of the display, which means everything from the timeline down cannot be accessed. Avidemux fits on a Ubuntu laptop with this resolution, so the problem appears to be related to the use of additional vertical space between the lines of text and/or objects in Fedora. (Ubuntu is no longer an option for me because of its display issue with VLC in 18.04 & 18.10)
There is a similar post from redtux1 dated 28 Mar 2013.
export QT_STYLE_OVERRIDE=fusion
avidemux3_qt5
By all means I would recommend to build from the current git (2.7.1 had two notable regressions).
Thanks eumagga0x2a, I tried the above code and it improved the scaling of the window to the point where the progress line and the navigation buttons are visible. The last line with Time, Frame Type and Selection are still off the bottom of the screen (and I now realise this was also the case with Ubuntu at this resolution), but Avidemux is now sufficiently workable for me on this laptop.
I also tried the Avidemux Appimage on this laptop, and this version does scale its window down nicely to fully fit on a 1366x768 display. It also worked fine once the installed version was used initially to populate its Open File dialog with the location of my network drive.
Further steps might be reducing font size (affects all other applications too) and hiding the toolbar in Avidemux (has to be repeated each time Avidemux is launched).
With AppImage some Hardware Acceleration features like video encoding in Hardware may be unavailable. Building Avidemux is trivial on Linux, I would recommend taking this route.
Thanks again eumagga0x2a, hiding the Codecs toolbar reduced the window size to fit within the display. (Depending on the video size, zoom may need to be toggled in and out to complete the window resize if the Codecs toolbar is viewed and hidden again.)
I am looking into the build option.
FYI: you can run your Avidemux build without installation: https://github.com/mean00/avidemux2/blob/master/run_avidemux_template.sh
# dnf builddep avidemux
Then as user, assuming git is already installed:
git clone https://github.com/mean00/avidemux2.git
cd avidemux2
bash bootStrap.bash --with-system-libass --with-system-liba52 --with-system-libmad --with-system-libmp4v2 --rebuild
HTH