How can I wrap long text strings in tmap HTML polygon popups or fixed widths? Can tmap recognize line breaks?

So to address question 1 I've made an example using a single point as a slightly simplified example, where I set some coords and also make up some sample columns to include in labels.

tibble(
  lon = 149.068778, 
  lat = -35.230741, 
) %>% 
  st_as_sf(
    coords = c(1:2), 
    crs = 4326
  ) %>% 
  mutate(
    city = 'Canberra', 
    country = 'Australia', 
    weather = 'sunny'
    ) %>% 
  {. ->> my_popup}

my_popup 

# Simple feature collection with 1 feature and 3 fields
# Geometry type: POINT
# Dimension:     XY
# Bounding box:  xmin: 149.0688 ymin: -35.23074 xmax: 149.0688 ymax: -35.23074
# Geodetic CRS:  WGS 84
# # A tibble: 1 x 4
#               geometry city     country   weather
# *          <POINT [°]> <chr>    <chr>     <chr>  
# 1 (149.0688 -35.23074) Canberra Australia sunny  

Option 1 - tell tmap what info to show on each line in the popup

This is pretty simple, and doesn't rely on any HTML coding or specifying line breaks. However for this same reason, it's probably reasonably specific to tmap and a bit different to how you might do this in leaflet, or how you would make labels in ggplot. Basically you tell tmap what to show in the labels using popup.vars, and supply a "line title" and the data for that line.

tm_shape(my_popup)+
  tm_dots(size = 1,
          popup.vars = c('City: ' = 'city', 
                         'Country' = 'country', 
                         'Weather: ' = 'weather')
          )

The "line titles" (left hand side of the =) can be whitespace, but not empty; ' ' is ok but '' is not ok.

Option 2 - Format the label in HTML

Make a new column in my_popup, called label. This is a single text string, written in HTML style (this is a common way to make custom popups in leaflet). It can include text strings, line breaks (<br>), values from columns in the data and even a URL link with custom hyperlink name (and I assume other HTML stuff).

my_popup %>% 
  mutate(
    label = str_c('First line of my label <br>',
                  country, '<br>',
                  'Weather: ', weather, '<br>', 
                  'Link to website: <a href = "https://visitcanberra.com.au/"> Custom link name </a>'
                  )
  ) %>% 
  {. ->> my_popup_1}

my_popup_1

# Simple feature collection with 1 feature and 4 fields
# Geometry type: POINT
# Dimension:     XY
# Bounding box:  xmin: 149.0688 ymin: -35.23074 xmax: 149.0688 ymax: -35.23074
# Geodetic CRS:  WGS 84
# # A tibble: 1 x 5
#               geometry city     country   weather label                                                                                                                                         
# *          <POINT [°]> <chr>    <chr>     <chr>   <chr>                                                                                                                                         
# 1 (149.0688 -35.23074) Canberra Australia sunny   "First line of my label<br>Australia<br>Weather: sunny<br>Link to website: <a href = \"https://visitcanberra.com.au/\"> Custom link name </a>"

Then, we can specify the label column as the (only) popup.var in tm_dots(). In order for tmap to acknowledge the HTML style, we must tell it NOT to escape HTML using popup.format. By default, html.escape = TRUE; see ?tm_dots for explanation.

tm_shape(my_popup_1)+
  tm_dots(size = 1,
          popup.vars = 'label',
          popup.format = list(html.escape = FALSE)
  )

Notice that tmap automatically includes the name of the column where the popup info comes from as grey text on the left hand side of the popup - in this case our label column. This is unnecessary in our somewhat complex and self-explanatory label, but might be handy if you had a single value in the popup.

To get rid of this, we specify the "line title" of the popup in popup.vars as we did in Option 1, but this time as whitespace (whitespace seems to be ok, empty string not ok).

tm_shape(my_popup_1)+
  tm_dots(size = 2,
          popup.vars = c(' ' = 'label'),
          popup.format = list(html.escape = FALSE)
  )

Or, you can somewhat combine the two methods to include HTML (eg the hyperlink), and also "line titles" (including empties):

my_popup %>% 
  mutate(
    line_1 = 'First line of my label', 
    line_2 = country,
    line_3 = str_c('<a href = "https://visitcanberra.com.au/"> Custom link name </a>'),
    ) %>% 
  {. ->> my_popup_2}

my_popup_2

# Simple feature collection with 1 feature and 6 fields
# Geometry type: POINT
# Dimension:     XY
# Bounding box:  xmin: 149.0688 ymin: -35.23074 xmax: 149.0688 ymax: -35.23074
# Geodetic CRS:  WGS 84
# # A tibble: 1 x 7
#               geometry city     country   weather line_1                 line_2    line_3                                                              
# *          <POINT [°]> <chr>    <chr>     <chr>   <chr>                  <chr>     <chr>                                                               
# 1 (149.0688 -35.23074) Canberra Australia sunny   First line of my label Australia "<a href = \"https://visitcanberra.com.au/\"> Custom link name </a>"

tm_shape(my_popup_2)+
  tm_dots(size = 1,
          popup.vars = c('Line 1: ' = 'line_1', 
                         ' ' = 'country', 
                         'Weather: ' = 'weather',
                         'Link to website: ' = 'line_3'), 
          popup.format = list(html.escape = FALSE)
  )

So, Option 1 is pretty simple, but if you want to get fancy you will probably want to use Option 2 to take advantage of the features of HTML, including text colours and alignment (especially as it looks like the popups are aligned to the right by default, which may not suit).

For example, text colour:

my_popup %>% 
  mutate(
    label = str_c('<p style="color:red">This is my label</p>')
  ) %>% 
  {. ->> my_popup_3}

tm_shape(my_popup_3)+
  tm_dots(size = 1,
          popup.vars = c(' ' = 'label'),
          popup.format = list(html.escape = FALSE)
  )

Or adjust the alignment:

my_popup %>% 
  mutate(
    label = str_c('<p align="left">This is my long label<br>More<br>More</p>')
  ) %>% 
  {. ->> my_popup_4}

tm_shape(my_popup_4)+
  tm_dots(size = 1,
          popup.vars = c(' ' = 'label'),
          popup.format = list(html.escape = FALSE)
  )

my_popup %>% 
  mutate(
    label = str_c('<p align="left">This is my long label<br>More<br>More</p>', 
                  '<p align="right">More<br>and<br>More</p>')
  ) %>% 
  {. ->> my_popup_5}

tm_shape(my_popup_5)+
  tm_dots(size = 1,
          popup.vars = c(' ' = 'label'),
          popup.format = list(html.escape = FALSE)
  )

In terms of question 2, rather than "modify the tmap popup options", I would suggest you would move the layers to leaflet rather than tmap, and adjust popups using HTML as above. Hopefully this isn't required with the above solutions, but you'd use something like:

tm_leaflet(my_tmap) %>% 
  addMarkers(popup = ~my_label)

Question 3 - scroll bars. I'm not sure how/if these can be implemented, I have a feeling I've seen them when I have included a large image in the popup (using HTML), which must trigger a maximum popup size. Hopefully the 2 options above sort you out enough you don't need scroll bars anyway.