Fix Panel incorrect size

[zhatuokun]

• Closes SidePanel::rights with an exact_width have incorrect padding #4126
• Closes SidePanel::exact_width does not product a panel of that size #3006

Previously, SidePanel/TopBottomPanel didn't include inner_margin when setting the min width/height of Frame. As a result, when your Panel has inner_margin, its size doesn't work as expected.

Before

After

[emilk]

Looks good, but please also update the docstrings of the min_height, max_height, height_range etc to explain they refer to the inner size, excluding the margins.

[zhatuokun]

Looks good, but please also update the docstrings of the min_height, max_height, height_range etc to explain they refer to the inner size, excluding the margins.

Thanks! Previously, if you called SidePanel::min_width to set the minimum width to 150.0, the previous algorithm would result in the SidePanel's actual minimum width being 150.0 + inner_margin.left + inner_margin.right. Under the default margins, this would add an extra 16.0 in size, for a final SidePanel actual minimum width of 166.0. Therefore, I subtract their inner_margin, meaning that when you use min_width to set the Panel's minimum width to 150.0, the Panel's actual size is 150.0, and the content inside is 150.0 - (inner_margin.left + inner_margin.right), which is consistent with the documentation description (Minimum width of the panel.). The same applies to other properties.

[emilk]

The problem with the existing documentation (and this is my mistake) is that "Minimum width of the panel" is not at all clear wether it is the outer size (size including margins) or the inner size (excluding the margins).

It seems the existing code was also confused, hence the bug. Bugs will keep cropping up unless we define explicitly wether or not the size includes the margins.

[zhatuokun]

That's true, for this aspect, perhaps we can refer to the design of CSS box-sizing, but that's another issue. For now, due to the two issues I mentioned above, I suggest merging the changes after I add the document. I will start working on it soon.

[emilk]

Thanks!