Massive window I can't reduce in size - how to alter the "Program supplied minimum size"

Question

I have a window (TortoiseHg) that whenever I open it is massive and I can't reduce the size. I know about Alt-drag and I can (eventually) get to the edge, but I can't reduce the size. If I run xwininfo -all the information includes:

xwininfo: Window id: 0x7200005 "TortoiseHg Workbench"

  Root window id: 0x192 (the root window) (has no name)
  Parent window id: 0x111e83a (has no name)
     1 child:
     0x720000e (has no name): ()  1x1+-1+-1  +4271+546

  Absolute upper-left X:  4272
  Absolute upper-left Y:  547
  Relative upper-left X:  1
  Relative upper-left Y:  28
  Width: 16383
  Height: 16383
  Depth: 24
  Visual: 0x139
  Visual Class: TrueColor
  Border width: 0
  Class: InputOutput
  Colormap: 0x7200004 (not installed)
  Bit Gravity State: NorthWestGravity
  Window Gravity State: NorthWestGravity
  Backing Store State: NotUseful
  Save Under State: no
  Map State: IsViewable
  Override Redirect State: no
  Corners:  +4272+547  --15135+547  --15135--15850  +4272--15850
  -geometry 16383x16383+4272+547

  ...

  Normal window size hints:
      Program supplied minimum size: 16383 by 16383
      Program supplied window gravity: StaticGravity
  No zoom window size hints defined

  No window shape defined
  No border shape defined

In particular this line:

      Program supplied minimum size: 16383 by 16383

I have tried:

• various ways of "maximising" the window, but none of them appear to have any effect. (The same methods work on other windows fine.)
• removing ~/.config/TortoiseHg/ and re-running
• searching for files in ~/.cache/ or ~/.local/share/ for something that might cause this
• searched dconf and gconf using this technique for any keys referring to thg or tortoise (case insensitive grep)
• running with strace -f thg looking for files it is opening that might set the minimum size

But I have not found anything that tells me where the "Program supplied minimum size" might be coming from. Anyone know?

System details:

• I'm using Ubuntu 18.04 with the Ubuntu Mate session, installed via the ubuntu-mate-desktop package.
• TortoiseHg is installed from the standard repos for 18.04 - no PPA etc. The version reported by apt is 4.5.2-0ubuntu1 and thg version reports TortoiseHg Dialogues (version 4.5.2), Mercurial (version 4.5.3)
• The system is a laptop with 2 extra monitors attached via a USB-C docking station. I have disabled HiDPI/scaling options.