Seems like it always worked this way:

Marking as:
Minor: Can slow down professional quality work but will not prevent (undo possible);
High: Makes the sidebar spacing much less functional + regression + common task


Missed a step in instructions:
1) write a word;
2) highlight word;
3) Put cursor in sidebar paragraph spacing (above or below) input field;
4) change value to 1;
5) push enter

So it seems like Writer is treating it like the cursor is not in the sidebar but instead is just in the document but this isn't the case, the cursor actually is obviously in the sidebar as you can see it blinking in the sidebar so IMHO nothing you do there should be able to completely delete text.

This being said, it's fair for UX to decide, feel free to close if you don't agree.

Confirmed this is a bug that goes all the way to when the sidebar was implemented.

Version: 5.2.0.0.alpha0+
Build ID: ed51d4293dd919a03edca11ec48c607bbfa31076
CPU Threads: 2; OS Version: Linux 4.2; UI Render: default; 
TinderBox: Linux-rpm_deb-x86@71-TDF, Branch:master, Time: 2016-03-07_00:33:34
Locale: en-US (en_US.UTF-8)

Steps:
1) Open Writer
2) In sidebar select a combobox (e.g. Styles) or spinbox (e.g. paragraph spacing above) and press enter
3) Notice that you Undo is now enabled in the toolbar and an new line was added in the document

Hmm, Linux only?

Can not confirm. On Windows 8.1 Pro 64-bit en-US with
Version: 5.2.0.0.alpha0+
Build ID: 7ccdb94e2c5774f924bf89b34387c7d41e2e4c30
CPU Threads: 8; OS Version: Windows 6.29; UI Render: GL; 
TinderBox: Win-x86@39, Branch:master, Time: 2016-03-03_02:40:45
Locale: en-US (en_US)

If I highlight paragraph and in the Sidebar Paragraph spacing--above or below-- enter 1 the spacing becomes 1.00 inch.  If entered in both above and below, the two selected one word paragraphs will reformat-- 1.00" above the first, 2.00" separating the two, and 1.00" below the second.

This is behaving as I'd expect--although entering 1 is a rather unuseful value. The spinner widget is increasing and decreasing the values--for me in 2/100" increments. Also, the keyboard <Tab> and <Shift>+<Tab> movement between the button widgets in the content panel seem to be behaving. And <F10>, <F6> for focus navigation back to the document canvas is behaving as well.

(In reply to V Stuart Foote from comment #4)
> Hmm, Linux only?
> 
> Can not confirm. On Windows 8.1 Pro 64-bit en-US with
> Version: 5.2.0.0.alpha0+
> Build ID: 7ccdb94e2c5774f924bf89b34387c7d41e2e4c30
> CPU Threads: 8; OS Version: Windows 6.29; UI Render: GL; 
> TinderBox: Win-x86@39, Branch:master, Time: 2016-03-03_02:40:45
> Locale: en-US (en_US)
> 
> If I highlight paragraph and in the Sidebar Paragraph spacing--above or
> below-- enter 1 the spacing becomes 1.00 inch. 

Push enter after entering the number 1.

Oops, nevermind. It afflicts Windows builds also.

Just read Jay's note.  And, entering "Enter" key while in the Paragraph content panel spacing spinners widgets does have a linkage to the selected text in the canvas. The "Enter" key action "inserts" blank text into what had been highlighted.

The Indent spinner widgets also do the same this of clobbering the selected paragraph with an Enter key press.

Related patch by Jim: https://cgit.freedesktop.org/libreoffice/core/commit/?id=bcfc348663b04834fe8e6d6315f647be1595469a

Created attachment 144898 [details]
video

Please see the video. 

In video I higlighted the text, then I changed the space to 1 and press ENTER several times. It seems it works ok.


Version: 6.2.0.0.alpha0+
Build ID: 9a9b81c7212fa6a6762246593acf3f1950677a22
CPU threads: 4; OS: Linux 4.15; UI render: GL; VCL: gtk2; 
TinderBox: Linux-rpm_deb-x86_64@86-TDF, Branch:master, Time: 2018-09-08_00:00:43
Locale: ro-RO (ro_RO.UTF-8); Calc: threaded