Unfortunately, Simplemind Pro (just 30 bucks, so paying for it in case would not have been the problem) is a strict, graphic outliner (people erroneously call’em “mind map” tools: good night if your mind was THAT simple indeed…), i.e. does NOT allow elements, let alone clusters of elements, independent of the graphic’s central (i.e. root) element, so it’s not a Scapple alternative.
Which is an incredible pity indeed since in the Windows desktop non-subscription world at least, there is not a single similar program (even at tenfold the price), coming with a Notes pane (as SimpleMind Pro does, see the first screenshot above), and which is necessary in such a graphic software (years ago, the name was “flowcharters”, today it would rather be “concept maps”), in order to preserve a neat “map”, graphic, with additional info readily available upon selection of the element or just “mouse over”: What had been perfectly possible with Mixrografx Flowcharter 2000 more than 25 years ago, has simply become unavailable in “flowcharters” since, and e.g. Scapple developers prefer to “hold it simple”, I was told: obviously, this means their code base, and not their customers’ canvases, but then, my suggestions for streamlining Scapple-user interaction, here in this forum, have never been addressed either which puts the forum thread “Is Scapple still being developed or supported?” in another, dire perspective, whatever people may say in there re cars not becoming flying saucers in time not meaning there’s no development.
Thus I trialed SimpleMind Pro, and its Notes pane works as expected (its too tiny font can be resized (not in the Font settings but) in the pane’s settings), but those so-called (=strictly-hierarchical, whatever their “free” lay-out on the canvas may fallaciously indicate otherwise) “mind maps” are invariably limiting your mind…
As is Scapple, by relegating any additional ideas / details / whatever there is which would clutter your graphic, to (at the very least) some second software (e.g. Scrivener, and then happy forth-and-back between the two, again and again and again, even in (iteratively occurring, no “waterfall”) conceptual stages in-between writing, in other words:
Current Scapple forces you to separate elements which you should hold together* so that in your mind, they will NOT fall apart… and thus partly in oblivion - or you will cramp the canvas in a way that you’ll again and again miss parts of the wood for sheer mixup of trees.
*=The (lesser) alternative to a Notes pane (on-off by toggle, obviously, and empty-filled = automatic by existence or non-existence of additional text for the selected / hovered-on canvas element) having been the implementation of “lavels”, whatever you call them: (sufficiently-sized and immediately-editable!) text boxes appearing upon hovering over an element (here again with resizable, and persistent for all these boxes then, font size)…
but here again I was told (obviously code) simplicity was the marching orders, not optimized, really smooth interaction between user’s mind, screen and user’s hand(s).
Scapple, Visio et al. fall behind in functionality, even to software available a whole (human) generation ago, and their developers get away with it because fanboys, instead of putting a “+1” beneath legitimate requirements, ridicule’em as being not only irrelevant but out of the question, see the flying cars in the thread mentioned above.
Btw, Scapple’s (fixed = invariable) link format to third-party files prevents the user from creating “deep links”, from Scapple canvas elements to specific items, pages, paragraphs, etc within (sic!) third-party text files (instead of just to the file in whole), even if those needed “deep links” observe the acknowledged standard formats - this may be different for items within “Scrivener” though, I haven’t tried this, Scrivener not being my preferred writing software, for some functionality lacking, and having been lacking for many years. Which is a pity twofold, especially considering its extended user base, for one, that should enable some development, and, second, so many writers’ output would stand stand to gain… 
EDIT
It just occurs to me that I could mention an interesting variant, seen (more or less) in “Ron’s Data Edit”: There, the (csv) cells don’t have “text a”, and then, in a Notes panes, there is additional “text b”, but you put the whole text into the cell - directly or via the Notes pane in case -, but (according to the cell formatting) only the very first line of the text is seen within the cell.
Now, in Scapple (or similar programs), the developer might present a setting (=toggle at the user’s discretion) that the canvas elements only display their text up to the very first newline, AND - that might be a little bit demanding code-wise though? - the elements’ respective sizes will be adjusted to that very first part of the text… whilst the Note pane then will (as in Ron’s Data Edit) always display the whole text, i.e. including the very first line of it (“line” being understood as the part before the very first newline, as said, and which might be more than one technical “line” within the element itself, due to its word wrap according to the element’s size and the line length).
Thus, Scapple’s canvas would display the elements with their (more or less detailed) “title” line, whilst the Note field would repeat that “title” - i.e. NO empty Note pane anymore, for any canvas element -, and the full text in case; obviously (and as in the above-mentioned applications), the Note pane should be resizable, so that the user could decide if they just want to see the canvas, or just some additional text, or quite some additional text, to the detriment of their canvas size - according to their needs, and to their screen size/numbers, obviously.
I personally think this would be the most elegant solution… or would have been if anybody who develops that kind of (desktop, “Windows” Scapple-like) software had currently implemented such an “additional text in place, but not in the canvas” functionality.
