These issues appear to occur with a High DPI monitor with Viewer UI scaling or Windows UI Scaling:
  1. Cursor disappears in any text box, including on login screen, script window, search boxes, and chat windows.
  2. Shift-tab to un-indent, or pressing enter to auto-indent to next line in script editor is awkward. Shift-tab pulls more lines than it should back.
  3. Shift-Up to select a prior line off by 1 space making things misalign.
  4. Trying to shift-tab a single line erases it.
These may seem unrelated but I've had the issue occur over UI Scaling as well, so I avoid that now. Well, now Windows is UI scaling, and the same issue occurs.
Temp fix:
Properties on the viewer shortcut, Compatibility tab, Changed High DPI Settings, and Override high DPI to System. This slightly blurs fonts (ClearType is no longer active) but fixes the cursor issue. The text editor issue Ive seen occur from using any viewer UI scaling and trying to edit scripts.
If there was any doubt about it being a High DPI thing, my main monitor is 2560x1600 w/ 150% scaling in Windows. My secondary is 1920x1080 with 100% scaling, and just moving the window from main to secondary fixes the cursor issue.
In the mp4 added:
00-08s: I cant see my mouse so I'm moving it around between the UI and Input field showing it disappearing. This doesn't show in the recording.
08-12s: I pressed enter at the end of the llSay line to make a new line, and pressed Shift-Up. It should have selected right before llSay, but catches an extra space in front of it.
15-17s: Using keyboard again (cant see the mouse after all) copy several llSay lines and then select lines 4-8, Shift-Tab affects lines 4-12. Pressing again selects even more, and Shift-Tab operates on those as well.
17-27s: Undoing the extra lines to restore to new script default
27-32s: Select line 4 and Shift-Tab. It erases the line entirely. Undo, End, Shift-Home, Shift-Tab, does it again.