Unexpected rounding in field calculator (documentation needed)

[jgrocha]

Author Name: Alister Hood (@AlisterH)
Original Redmine Issue: 5153
Affected QGIS version: master
Redmine category:vectors
Assignee: Nathan Woodrow

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(possibly related to #13455)
If I do this I get the expected result: @4/2@
If I do this I get the expected result: @0.5@
If I do this the result is 0, regardless of the precision of the output field!: @2/4@
If I do this the results are shown with many decimal places, regardless of the precision of the output field!^: @$x@

^ However, when the layer is saved the results are rounded to the expected precision. N.B. this is the same as #13665

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Related issue(s): #15625 (relates), #19495 (duplicates), #19757 (duplicates), #21296 (relates)
Redmine related issue(s): 6353, 11182, 11487, 13235

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