System Information
Operating system: Darwin-18.7.0-x86_64-i386-64bit 64 Bits
Graphics card: Intel HD Graphics 5000 OpenGL Engine Intel Inc. 4.1 INTEL-12.10.12

Blender Version
Broken: version: 2.81 (sub 5), branch: master, commit date: 2019-09-02 20:51, hash: rBd74cc6c08c42
Worked: (optional)

Short description of error
When opening a tool menu (with spacebar) especially a long one such as in sculpt tools, if the bounding height would exceed the current viewport bottom edge, the menu will be moved upward (other angles may apply). This causes the menu to select a tool instantly. For example, if you have the draw tool selected, and menu the box at the bottom of the screen, then you will have scrape/mask/something else under the cursor, and it will re-select by logic (you released spacebar with a different menu item than the active item). This is a bug because the mouse didn't move, so that can't be the intended behavior.

Video example of the bug:
https://youtu.be/9pKdN_sqQws
secondary (longer) example, using 2.80 stable
https://www.youtube.com/watch?v=6rRTrpWNj34

devtalk thread with issue (moderator verified)
https://devtalk.blender.org/t/spacebar-instantly-picking/9347/4

Exact steps for others to reproduce the error
The default startup file is sufficient to reproduce the error.
Go into sculpt mode, move the mouse toward the bottom of the screen and press spacebar.