ONLY reporting because the crash report told me to.
---------------------------------------------------

Description:

>> GIMP crashed on navigating drop-down menus. Clicked on transparency option.

Additional info:
* package version(s)
* config and/or log files etc.
* link to upstream bug report, if any

>> CRASH REPORT AS FOLLOWS:
```
GNU Image Manipulation Program version 2.10.20
git-describe: GIMP_2_10_18-332-g68d21512bf
Build: unknown rev 0 for linux
# C compiler #
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-pc-linux-gnu/10.1.0/lto-wrapper
Target: x86_64-pc-linux-gnu
Configured with: /build/gcc/src/gcc/configure --prefix=/usr --libdir=/usr/lib --libexecdir=/usr/lib --mandir=/usr/share/man --infodir=/usr/share/info --with-bugurl=https://bugs.archlinux.org/ --enable-languages=c,c++,ada,fortran,go,lto,objc,obj-c++,d --with-isl --with-linker-hash-style=gnu --with-system-zlib --enable-__cxa_atexit --enable-cet=auto --enable-checking=release --enable-clocale=gnu --enable-default-pie --enable-default-ssp --enable-gnu-indirect-function --enable-gnu-unique-object --enable-install-libiberty --enable-linker-build-id --enable-lto --enable-multilib --enable-plugin --enable-shared --enable-threads=posix --disable-libssp --disable-libstdcxx-pch --disable-libunwind-exceptions --disable-werror gdc_include_dir=/usr/include/dlang/gdc
Thread model: posix
Supported LTO compression algorithms: zlib zstd
gcc version 10.1.0 (GCC)

# Libraries #
using babl version 0.1.78 (compiled against version 0.1.78)
using GEGL version 0.4.24 (compiled against version 0.4.24)
using GLib version 2.64.4 (compiled against version 2.64.3)
using GdkPixbuf version 2.40.1 (compiled against version 2.40.0)
using GTK+ version 2.24.32 (compiled against version 2.24.32)
using Pango version 1.46.0 (compiled against version 1.44.7)
using Fontconfig version 2.13.91 (compiled against version 2.13.91)
using Cairo version 1.17.3 (compiled against version 1.17.3)

```
> fatal error: Aborted

Stack trace:
```
/usr/lib/libgimpbase-2.0.so.0(gimp_stack_trace_print+0x3b9)[0x7f9c7e44e729]
gimp-2.10(+0xd45e4)[0x55fa59d6e5e4]
gimp-2.10(+0xd4a48)[0x55fa59d6ea48]
gimp-2.10(+0xd50db)[0x55fa59d6f0db]
/usr/lib/libpthread.so.0(+0x14960)[0x7f9c7d448960]
/usr/lib/libc.so.6(gsignal+0x145)[0x7f9c7d2a9355]
/usr/lib/libc.so.6(abort+0x127)[0x7f9c7d292853]
/usr/lib/libc.so.6(+0x25727)[0x7f9c7d292727]
/usr/lib/libc.so.6(+0x34936)[0x7f9c7d2a1936]
/usr/lib/libX11.so.6(+0x40c59)[0x7f9c7d154c59]
/usr/lib/libX11.so.6(+0x40cfe)[0x7f9c7d154cfe]
/usr/lib/libX11.so.6(_XEventsQueued+0x52)[0x7f9c7d154ff2]
/usr/lib/libX11.so.6(XPending+0x62)[0x7f9c7d146992]
/usr/lib/libgdk-x11-2.0.so.0(+0x572a9)[0x7f9c7df502a9]
/usr/lib/libglib-2.0.so.0(g_main_context_check+0x1da)[0x7f9c7d5ecdda]
/usr/lib/libglib-2.0.so.0(+0xa010b)[0x7f9c7d63d10b]
/usr/lib/libglib-2.0.so.0(g_main_loop_run+0x83)[0x7f9c7d5edc03]
gimp-2.10(app_run+0x38e)[0x55fa59d6dd2e]
gimp-2.10(main+0x39a)[0x55fa59d6d5ca]
/usr/lib/libc.so.6(__libc_start_main+0xf2)[0x7f9c7d294002]
gimp-2.10(_start+0x2e)[0x55fa59d6d75e]

```

Steps to reproduce:
create an unsaved image with multiple layers.
Have multiple Firefox windows open (6).
Attempt to modify the transparency/colour mode of the selected layer.