Stretched scaling in Jazz Jackrabbit 2 in windowed mode (fshack)

When playing Jazz Jackrabbit 2 in windowed mode, Proton in gamescope crops away the titlebar, menubar and border, which is great. But then it gets the scaling/cropped window size wrong, and it stretches the game pixels. For example when running the game at 640x480 it gets stretched to 653x531. Here is a screenshot with integer scaling, to prove that it is not scaling properly (tested on Steam Deck):

Screenshot from the game:

For reference, when running JJ2 on a GNOME desktop, you can see the menus and border:

The game has a fullscreen (DirectDraw) mode, but it seems to draw more power on the Deck so I would prefer to run it windowed (and fshack crops and scales it up with gamescope).

Proton logfile with WINEDEBUG=+fshack,+x11drv: steam-10702214918549012480.log

Some notable lines:

013c:trace:x11drv:X11DRV_create_win_data win 0x20082/1600003 window (0,0)-(653,531) whole (0,0)-(653,531) client (0,0)-(653,531)
013c:trace:fshack:fs_hack_monitor_from_rect rect (0,0)-(653,531)
013c:trace:fshack:fs_hack_monitor_from_hwnd hwnd 0x20082 rect (0,0)-(653,531)

Jazz Jackrabbit 2 TSF running with the Plus patch, installed from GOG, running on proton-7.0-2.

I first posted an issue to gamescope, but I think I found out that it is fshack inside Proton that's cause it, or gamescope getting the wrong size. https://github.com/Plagman/gamescope/issues/475

The report points to Proton's fshack path and includes a WINEDEBUG=+fshack,+x11drv logfile; begin by tracing the reported 653x531 window dimensions and compare them with the linked gamescope issue. Done means the 640x480 window is cropped and scaled without stretching in windowed mode.