I’d really like Select Sample Text window to stay open and allow editing. I’m finding the text samples extremely useful for kerning. But what’s frustrating is I have to select the text string, close window, kern, reopen, select text, close window, kern, etc., etc., etc.
In the meanwhile, in Glyphs 2, you could assign keyboard shortcuts to Edit > Other > Select Next Sample String and Edit > Other > Select Previous Sample String. That would allow you to switch to the next/previous sample string with a key combination. Also mentioned at the end of section 3.8.1 Sample Texts of the Glyphs 2.3 Handbook.
You can do so via the macOS System Preferences: go to Keyboard > Shortcuts > App Shortcuts > Glyphs and add a new shortcut using the + button.
But the default shortcut + arrow up/down + enter can be done very quickly. Having the window open means you need to grab the mouse to click the next row and that is most certainly slower.