When you enter one taxable transaction, you can't just watch the monitor. You increased your overall adjusted gross income and with that come many other changes in your return, not just the incremental tax on the one transaction.
Like for example it increased your AGI and that would decrease some deductions if you itemized on Schedule A. And by increasing your AGI it might reduce some credits you were getting like EIC. And can make more of any Social Security taxable.
If you have capital gains or qualified dividends the tax is not taken from the tax table but is calculated separately from schedule D. The tax will be calculated on the Qualified Dividends and Capital Gain Tax Worksheet. It does not get filed with your return. In the online version you need to save your return as a pdf file and include all worksheets to see it.
For the Desktop version you can switch to Forms Mode and open the worksheet to see it. Click Forms in the upper right (upper left for Mac) and look through the list and open the Qualified Dividends and Capital Gain Tax Worksheet.
