[Nerixyz]

If maximum <= minimum, then we would divide by 0.0000001. This could result in values for the y-position and height of the scrollbar rectangle that don't fit into a 32-bit signed integer. Consider the following:

• We're in a search popup without a search - all messages are displayed, we're at the bottom.
• currentValue_ = 800, pageSize_ = 20, trackHeight_ = 500
• Now, we're searching something, so the channel gets cleared.
• During this, maximum_ and minimum_ get set to 0 (while the rest stays as is)
• Then, calculating the y-position: $\frac{800}{\max(0.0000001, 0)} \cdot 500 + 1 = \frac{800}{0.0000001} \cdot 500 + 1 = 8000000000 \cdot 500 + 1 = 4000000000001 > 2147483648$

The difference between maximum and minimum is the number of messages currently in the channel. With this PR, we assume there's always at least one message when calculating the thumb rect. If there are no messages in a channel, we'll have an incorrect rect, but this won't be shown, because the scrollbar is always hidden in that case.

I updated the documentation of Scrollbar to better indicate the values and meaning of minimum and other members.

[Wissididom]

Changelog: cmputation