Set z-index for sticky diff box lower

[zeripath]

Fix #12525

As per @CirnoT comments on that issue.

Signed-off-by: Andrew Thornton art27@cantab.net

[codecov-commenter]

Codecov Report

Merging #12537 into master will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #12537      +/-   ##
==========================================
- Coverage   43.50%   43.50%   -0.01%     
==========================================
  Files         642      642              
  Lines       71003    71003              
==========================================
- Hits        30892    30891       -1     
+ Misses      35105    35103       -2     
- Partials     5006     5009       +3     

Impacted Files	Coverage Δ
modules/indexer/stats/queue.go	52.94% <0.00%> (-23.53%)	⬇️
modules/indexer/stats/db.go	43.47% <0.00%> (-8.70%)	⬇️
modules/git/utils.go	73.77% <0.00%> (-3.28%)	⬇️
modules/git/repo.go	49.23% <0.00%> (-0.51%)	⬇️
services/pull/pull.go	41.57% <0.00%> (-0.47%)	⬇️
modules/queue/workerpool.go	60.00% <0.00%> (+1.22%)	⬆️
modules/log/event.go	59.43% <0.00%> (+1.88%)	⬆️
modules/util/timer.go	85.71% <0.00%> (+42.85%)	⬆️

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