[stable12] Allow to close sidebar for text files

[MorrisJobke]

backport of #6499

Tested and works here 👍

[codecov]

Codecov Report

Merging #6525 into stable12 will decrease coverage by <.01%.
The diff coverage is n/a.

@@              Coverage Diff               @@
##             stable12    #6525      +/-   ##
==============================================
- Coverage       53.77%   53.77%   -0.01%     
  Complexity      22527    22527              
==============================================
  Files            1383     1383              
  Lines           86498    86498              
  Branches         1328     1328              
==============================================
- Hits            46517    46515       -2     
- Misses          39981    39983       +2

Impacted Files	Coverage Δ	Complexity Δ
lib/private/Files/Cache/Propagator.php	94.93% <0%> (-1.27%)	16% <0%> (ø)
lib/private/Security/CertificateManager.php	91.08% <0%> (-1%)	39% <0%> (ø)
apps/files_trashbin/lib/Trashbin.php	72.28% <0%> (-0.25%)	136% <0%> (ø)
lib/private/Server.php	84.8% <0%> (+0.12%)	121% <0%> (ø)	⬇️

[nickvergessen]

Works, however if the text in the first line is long, the X will overlay the text and is not discoverable anymore.