toggling shift or alt while scrubbing produces weird/inconsist results

Alt
1. start scrubbing a number with no decimal places, e.g. 100
2. hold alt and scrub, you should see 100.1, 100.2, etc.
3. release alt, you'll continue to scrub from the current number in increments of 0.1, e.g. if you release alt at 101.7 and scrub, the next number will be 101.8.

Suggested behaviour: as long as the mouse button is down, remember what increment the use started scrubbing at use that when modifiers are released. In this case, the next number should be 102.7.

Shift
1. start scrubbing a number with no decimal places, e.g. 100
2. hold shift and scrub, you should see 110, 120, etc.
3. release shift, if you were at 200 (with shift down) you'll be at 110 now and the next number would be 111.

Suggested behaviour: continue from the number just before release shift. In this case the next number should be 201.

Reproduce the Alt and Shift scrubbing sequences described in the issue and locate the number-scrubbing entry point in the browser editor. Done means releasing a modifier while the mouse button remains down preserves the increment and continues from the value immediately before release for both modifiers.

Written by the indexing model from the issue text.