What's with the Old Strings Fix?

I found onStrFix() in zquest.cpp, which has these clues:

Fix: Old Margins
Fixing margins may cause strings that used to spill outside the textbox to instead be cut off. Are you sure?

Fix: Old Frame Size
This will fix the frame size of all strings. No visual changes should occur as the string width/height will be fixed, but the compat QR will also be unchecked.

That menu option is disabled by:

		fixtools_menu.disable_uid(MENUID_FIXTOOL_OLDSTRING,
			!(get_qr(qr_OLD_STRING_EDITOR_MARGINS)
				||get_qr(qr_STRING_FRAME_OLD_WIDTH_HEIGHT)));

in zquest.cpp