The flash is programmed to use "device fonts", meaning it uses your computer's native font rendering of Courier New. However this doesn't happen and it uses the default Ruffle font. The cutoff text is a side effect of not using the right font - the text ends up being too wide relative to the size of the textbox.

The reason this happens is that Ruffle is inherently not capable of using device fonts due to fundamental restrictions on what javascript is allowed to access. Ruffle's intended workaround for this is for the website to provide an additional "fonts swf" that provides the missing fonts, like so:

https://github.com/MrCheeze/homestuck-ruffle-tester/blob/gh-pages/homestuck-fonts.swf

Those two swfs collectively contain every device font that homestuck tries to use, although courier is by far the most important. It uses a pixel-perfect Courier New instead of a variable size font, because Ruffle's rendering of the font is very inaccurate otherwise. The ruffle config lines linked above will make it use the fonts from the swf whenever a device font was used originally. Feel free to just take these font swfs and ruffle config lines directly from my repo and use them.