Oh I see.
This isn’t really a bug, is it? That’s just how textboxes work, if you move your cursor while clicking on them you’re drag selecting text, and if that drag select is off of the end of the text it will select nothing and put the cursor at the end.
How bad of an issue is this? It would take some truly heroic hacking of Qt to get a different behavior here and possibly mess up some of the other text selection mechanics in the box.
Ah, true. Clicking the box selects the whole thing to be able to clear & replace it in one stroke, which is different than every other textbox which would just place the cursor where you clicked or at the end; this makes the clear & replace functionality fail if the mouse slightly moves during the process. It was driving me nuts while building, personally. Trying to move something 2 studs and it turns into 12, then 122. 
I’m not sure what the best UX here would be; perhaps a shortcut keybind to select the entire text, which avoids the drag behavior and readies it for replacing. Or a UI button that does the same.
I’ve heard other people want a shortcut to focus the grid snap increment box regardless of this issue. One of the things people miss from SBS. I’ll look into that.
Hello,
This bug is still frustrating me even today and I’ll be illustrating why I think this is a bug that needs addressing:
Action for reproduction: Click, hold, and move a pixel while holding. In the one bugged location, it unhighlights the selection.
Unintended behavior (bug):
Intended behavior (for example, in Properties):
Nice! Keeps its selection