I have ‘emulate 3 button mouse’ set up to use alt. I’d like to keep it that way if possible. I also need to draw boundary strokes to contain the grease pencil fill. Normally you’d draw with the left mouse button while holding alt, but that conflicts with the 3 button mouse emulation.
Is there any other way to draw a boundary stroke from the fill tool?
“emulate 3 button mouse” is a preference to change a shortcut.
If you encounter conflicts, you can simply remap shortcut for boundary stroke creation in Grease Pencil section of keymap configuration.
Fill Area tool does not have option to only draw boundary strokes, you can choose to fill an area delimited by boundary strokes only, by edit lines only or both, by default.
But left click correspond to area filling.
thanks for the reply. that helps. i’m having trouble finding which shortcut i need to disable to prevent the view changing when i press alt and LMB click to try to draw a boundary stroke with the fill tool, none of the ones i tried seem to have any effect. any pointers much appreciated!
You can change alt key to OS key for Emulate 3 Button Mouse option.
Or you can remap Boundary Lines shortcut to something else.
I would have loved to choose the first alternative: the OS key. But it seems it’s no longer available in 2.92 
I am having the same problem now in 2.92.
Which alternative did you go with? - If you don’t mind my asking.
Thank you Tomasz
DPC
Your screencapture looks like preference window of a 2.80.
This ability was introduced in 2.81.
Original commit says that is disabled for Windows 32 bits.
I can only confirm that is still available in my linux version of 2.92.
Maybe you are confusing releases. Maybe you are facing a problem that is platform specific.
That is very strange.
According to the file I exported with the Blender system info I am using “2.92.0”, in Windows 10.
I will try it in Pop OS Linux then.
The only peculiar thing about my Blender setup is that I installed it via Steam.
Update -
Correct! For some strange reason I do get that dropdown menu to choose Alt or OS-Key when ticking the Emulate 3 Button Mouse checkbox when in Linux Blender 2.92!!!
But not in Windows 10.
I have both installed via Steam.
I guess Microsoft is not very flexible with that key 
I am going to install Blender the regular way and see if that will help.
Nope! That did not help either
I’ll have to try the Alpha 2.93!
Still not! Even the Alpha 2.93 won’t show me that dropdown menu