v1.3.16.1-x64
I am trying to duplicate a similar behavior to Graphics Gale’s right-click and drag-for-rectangular selection functionality and the Shift key is beating me down. I cannot find a single method to reliably duplicate the behavior of right-clicking and dragging a selection marquee.
My workaround was to use RMB to click to pick a color selection (quick swap), and hold Shift (instead of the right-click mouse button) to temporarily select the marquee tool so that I can left-click to select the rectangular area. This failed spectacularly when I disabled the “Preview straght line immediately on pencil tool” option so that Shift wouldn’t keep trying to swap to the line tool. I simply wanted to use Shift for a quick Marquee tool selection. However, after about a click or two with Shift being held with this option disabled (especially when I set “Shift+D” as deselect to mimic the behavior of simply clicking outside of the rectangular selection to deslect in Graphics Gale), something weird happens and the tool stays being an “Additive” selection. No matter what I set the main tool to or even the quick select marquee tool to when holding Shift, the selection doesn’t create a rectangular selection – instead, it tries to “paint” the additive selection using the pencil tool as if it were painting in more to the selection and getting confused about the paint or selection marquee tool.
This is pretty frustrating as I really prefer the user-experience from Graphics Gale in terms of moment-to-moment pixeling. However, Aseprite has all the best features outside of the comfort of moment-to-moment pixeling that feels great. I’ve yet to adopt Aseprite fully and only coming back to it on very rare occasions when I need its feature set. I just hate that I can’t figure out how to replicate (or at least emulate) the Graphics Gale right-click selection mechanism.
Any help would be appreciated. It just makes Aseprite seem really buggy to be unable to replicate that sort of functionality despite seemingly having the features/ability to do so. 
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EDIT:
Follow-up. I downloaded the new Steam version (beta) and used the new mouse control shortcut setting for right-click and (even though the above selection mask mode bug still exists in the beta), I managed to replicate the Graphics Gale right-click select/dragging behavior far more precisely by using the Rectangle Marquee tool on right-click (while dragging).
Call me a happy camper!
EDIT 2:
Stumbled upon “Action Modifiers” menu accidentally and realized that “Action Modifiers > Selection: Add Selection” was set to Shift automatically (along with various other functions). Had no idea to look here at first. Apparently this “Add Selection” wasn’t a bug, but a feature. I was looking under the Rectangular Marquee tool for anything pertaining to that tool and modifier keys, but definitely did not realize there’s a whole other menu that didn’t even directly pertain to the tool, just selections in general (which happend to be interfering with my Shift key being used for the quick Marquee Select tool).
SUGGESTIONS:
As I was trying to replicate the basic behavior for Graphics Gale’s selection process (which helps immensely with the ergonomics of animation), I ran into these usability issues. Obviously the solution to the initial confusion would be to keep the shortcut modification options / functionality organized closer to the tools themselves.
Two versions of the basic tools seems to exist - the quick select tool and the regular one. The icing on the cake would be to let me move near the selection I made and the transformation handles would appear and change my tool (possibly with or without the press of a modifier key, configurable by the user) so that I can quickly click on the transformation handles to rotate or move the selection around. This would be a godsend for animation, as I currently need to fully select between the Rectangular Marquee tool and the Paint tool, which slows me down immensely (I don’t zip across my keyboard like many to press random letters to swap between tools – I like everything one-handed on my mouse and the modifier keys or the keys near them – I prefer to never lift my hand off the mouse when I am doing pixel art, using my left hand to handle any extraneous quick actions alone, with my right handling menu digging or pixel-clicking). D:
For manual animation processes, having an Action Modifier option to create/hide the selection/rotation controls while on the pencil tool (or quick-selecting the Rectangular Marquee / Selection tools) saves a ton of headaches in ergonomics, so it would be amazing to have Aseprite support that type of workflow! <3
EDIT 3:
Geeze. Call me blind! Thanks to the previous issue, I decided to look a bit closer. Apparently I had “Ctrl” bound to the Move tool shortcut by default. So the handles weren’t appearing when I would press Ctrl as they might in other programs, so I didn’t see a way to do any modifications to the selection after I selected it. When I put “Rectangular Marquee Tool (Quick)” to both Right-click and to the CTRL key, the transform/rotate/etc. handles appear when I press CTRL, which gives me the selection behavior in Graphics Gale.
Any chance you could add a Shortcut Profile for Graphics Gale so that beginners who come from other pixel art applications could set this up without having to know the nuances of the shortcut system at first? Simply selecting “Graphics Gale” as a shortcut/action modifier profile (which enables/disables certain other editor options to allow for that application’s behavior) would be massively more user-friendly to those of us who would love to use Aseprite, but who don’t realize just how deep, nuanced, and capable its underlying systems are!! - I was able to replicate Graphics Gale’s very specific pixeling workflows with just a bit of trial-and-error on the latest version! 
This post should not be a bug report, but a user-story. I do think Aseprite needs a Graphics Gale shortcut profile easily selectable by default, but aside from that, the only problem here is that I was fooled by Aseprite’s robustness (which resulted in seemingly bizarre behavior due to some unintutive-to-me defaults) into thinking the intended behavior was somehow buggy). As a software dev myself, I know this is par for the course so I dislike that I fell into the trap. Aseprite has impressed me more than I would’ve ever expected it to thanks to this experience (and the unexpectedly amazing features it sports under the hood that have always pushed pixel art tooling to its limits!)
Once again, I am amazed by and appreciate all your efforts @dacap! 