On Linux Mint 18.3, Musescore 3.4.2 and 3.5 RC display is too large for the physical display screen size.
On launching of Musescore AppImage for Linux, the windows are too large --- splash screen, start center, workspace, preferences, etc.. I have to use Alt-drag to move the window until I can see the bottom row of buttons, or see the window controls in upper right corner. I was able to resize the workspace window and simplify what's in it to make it useable, but other windows (such as preferences) refuse to resize below what they think is their minimum size even though the display is far larger than they need. This did NOT happen on version 2.0.2 for Linux on same desktop system. It does happen with 3.4.2 and the latest 3.5 RC that I just downloaded today. I had been hoping the problem would just go away with version 3.5, but alas, it's still there.
I'm running Linux Mint 18.3 Cinnamon 64-bit on an AMD Athlon II X2 desktop with Nvidia GeForce GTX 1050 using recommended Nvidia proprietary driver connected to a 1920x1080 display. This isn't a new system. It has been working fine with all other programs except Zoom app for Linux, which also does over-size, but running Zoom from Google Chrome browser has no problem.
Musescore 3.4.2 runs fine, without this oversize problem, on my HP laptop running Linux Mint 19 Cinnamon 64-bit on an AMD Ryzen 3 processor with AMD Radeon Vega display processor and a 1600x900 screen. Maybe the problem lies within Linux Mint 18.3. I hate to reinstall Linux, but now that Linux Mint 20 was released about 3 weeks ago, I'll give it a few more weeks for bugs to start settling out and then install it on the desktop system. I'll report the results of Musescore on this issue ticket when I do.