Unity Version 2020.1.0a1 (998f3306adcf)
Reproduces as far back as 2018.4.4f1 (5440768ff61c)

Repro steps
1. Open a shadergraph or VFX graph
2. Create any node that has a variable that is a constant by default (and has an input field with a number which you can select or deselect
3. Click on the right side of the number, and hold, move your mouse over to the left side of the number, to highlight it
4. while still holding, move your mouse down slightly, below the screen Y position of the bottom of the text box
5. Click on the left side of the number, and hold, move your mouse over to the right side of the number, to highlight it
6. while still holding, move your mouse up slightly, above the screen Y position of the top of the text box

Expected result: the number will remain highlighted after click-drag highlighting, no matter where you move your mouse, as long as it is on the other side of the number from where you originally clicked
Actual result: the number will be deselected if you move the mouse in the wrong vertical direction after highlighting the text or number.